Laser Kids
RUS

. : UNDIYING or LASER DIODES DEATh CHRONICLES : .

 

Why Un DIY ING? Just because the laser diode is a ready-made commercial grade device and DIY technologies can only be applied to create a power supply or a nice looking housing. Or, maybe, for the focusing optics (if it is really needed there).

On the power supply: It is relatively wide known already that one must feed the laser diodes using so called "constant current source" and not by a voltage regulator. The simplest are the analogue regulators. Here is a pair of good circuits.

 

Circuit N1

The circuit is based upon one chip voltage regulator being connected as a constant current source:

                                       ____________
                                      /     O      \
                                     |              |
                                      --------------
                                     |   KPEH22A    |
                                     |  LT1083-CT   |
                                     |              |
                                     |Adj  Out   In |
                                      -+----+-----+-
                                       |    |     | 
                                       |    +--+  |
                                       |       |  |            SW1  6 Amp
                               R2..R5  +-/\/\/-+  |               /
                               1.0 Ohm |       |  +---------+----+  +------+
                               each    +-/\/\/-+  |         |              |
                                       |       |  |         |    2 x 18650 |
                                       +-/\/\/-+  |         |     Li-Ion   |
                                       |       |  |         |      cells -----
  5W     +----->>--+-----------+-------+-/\/\/-+  |         |             ===
 808 nm  |         |           |  C1              | C2      | R6         -----
        _+_        / R1        |  >400uF          | >400 uF / 10 kOhm     ===
 Laser  \ /        \ 10 kOhm  --- LOW ESR        ---        \              |
 Diode  -+-        /          ---                ---        /              |
         |         |           |                  |         |              |
         +----->>--+-----------+------------------+---------+--------------+

 

Circuit_N1a

Circuit_N1b

 

Resistors R1 and R6 are only to discharge electrolytic caps C1 and C2 safely. The voltage here is always not higher than 10 volt, so the word "safely" does not mean any danger to Your health or life. On the contrary the charge of C1 and C2 is VERY dangerous for the life of the laser diode. In case of any random contact it can easily kill the precious device.

C2 capacitor serves for prevention of SW1 switch ringing.

C1 capacitor is of the very importance here. Calculations in LTSpice and modeling on real circuits does show that there exist a positive and non-zero value of the capacity of this capacitor, below which the circuit refuses to operate - it produces an over-regulating overshot current and kills the diode. This threshold capacity is individual and depends to the voltage regulator chip parameters as well as to the current setting. For most chips like LT1083 it is about 100-200 mcf. For LM317's this value is lower. So if we chose it to be C1>400mcf we should have enough of safety margin.

Resistors R2..R5 are current measuring/current setting ones. Generally the principle of operation is just to force the on-chip voltage regulator to set the constant voltage across the fixed resistor. Since the voltage is constant and the resistance is constant, the Ohm's law will unavoidably set the current to be constant too. The resistor is composed from many ones. There are three good reasons to do so:

  • They dissipate high power;
  • The errors of their resistance will partially compensate each other;
  • By varying of the number of soldered resistors one can vary the value of the current, being produced by the circuit. (And thus to set the power of the laser.)

Avoid to use a potentiometer here! It is a proven way to kill the laser diode. Avoid switches too. One can vary the number of the resistors by soldering only. In the worst case You can use computer jumpers. But only if You are sure that they are reliable enough.

 

Bonuses of the circuit:

This circuit is VERY simple and uses VERY affordable parts. Along with this it has a high precision and many of internal protections of any kind: from shortcircuit, from overheat, from overvoltage of the power supply and so on...

 

Maluses of the circuit:
Despite the comprehensive set of protections the circuit is NOT protected from the reverse polarity. So be extremely attentive when installing the batteries.

Another issue is that this circuit hates a bad connection between it and the laser diode. Indeed, when the connection is bad the constant current generator tries to rise the current and charges the C1 cap. And when the connection recovers, all the charge from C1 flows through the diode and kills it. This drawback is more or less the property of any constant current type regulator. Use reliable wires and soldering. One should never try any kind of "poking" of the leads of the laser.

 

The circuit is brakish enough. The typical rise and off time is well above one millisecond, and it is determined by charging the C1 and C2 caps. However remember that those caps are chosen exactly due to the properties of the chip itself. So we can say that the circuit is slow due to the fact that those voltage regulators are too slow themselves.

 

Another issue is that the circuit behaves badly under the dynamic changes of load. It is very expectable - there is no limits for the current to flow between the large capacity C1 and the laser diode. With powerful gallium arsenide diodes (red and infrared ones) there is nothing bad here, but with gallium nitride ones (blue and green) it may cause failures. The last ones behave like they have an area of negative differential resistance, like tunnel diodes.

 

One should also keep in mind that some of the on-chip voltage regulators are fake ones. When buying those LT1083, LM196, LM338 etc one can easily face something very wrong under the good looking outfit. So ALWAYS CHECK THE CORRECT OPERATION OF THE NEWLY ASSEMBLED CIRCUIT BEFORE CONNECTING LASER DIODE TO IT!

 

Tho check it out it is better to load it by different resistors and to look how the current depends to the load. Hint: if the circuit works correctly the current should remain almost constant with any reasonable variations of load (say between zero/shortcircuit and one Ohm).

 

If one has an oscilloscope it is very desirable to check the rise and fall curves occurring during turning the circuit on and off.

 

Generally the impression is very positive from this circuit. For better safety one should add a diode between the battery and the circuit - it would protect from the reverse polarity, and add a small resistor in series with the laser diode (like 0.5-1 Ohm) - it will make it more stable under the dynamic changes of load. However these improvements cause high overhead in the required size of the battery. Consider 1 additional volt across the protective diode and one or two volts across the stabilizing resistor.

 

The efficiency of the circuit grows with rising the voltage drop on the diode. So using this circuit to feed a blue diode (4.3 V) or to feed two IR diodes in series (4.8 V) one can expect the efficiency as high as 50%.

 

This circuit has:

  • Powered two portable 5W lasers for reasonably long time without a fail;
  • Burned out one red diode from DVD drive - a try to control current by a variable resistor;
  • Burned out one blue diode. It is when it was found out that with this circuit and the blue ones, one should not exceed a half of the current rating of the diode being used.

 

Circuit N2

Two transistors as constant current generator.

There are several variations based on the same idea:

 

NPN + MOSFET:

              /
          +--+  +---|>|-------+---------------------+
          | SW1    10A10      |                     |
          |                   |                     |
          |                   \ R1                  |
          |                   / 100 Ohm     irf3205 |
          |                   \                   |-+Drain
          |                   /              Gate||
          |                   |__________________J|-+
          |                   |             |     |-+Source
          |                   |Collector    |       |
          | 2 cells            \            |       |
          | Li-Ion       2n2222 \|Base      |       |
          | 18650               /|----------)-------+
          |                    /            |       |
        -----                 |Emitter  R2  \       \ R3
         ===                  |        10k  /       / 0.14 Ohm
          |                   |             \       \
          |                   |             /       /
        -----                 |             |       |
         ===                  +-------------+-------+
          |                                         |
          |                            +-----+------+ 5W
          |                            |  C1 |      | 808 nm
          |                        R4  \  10u|      | Laser
          |                        10k /    ---    _+_
          |                            \    ---    \ /
          |                            /     |     -+-
          |                            |     |      |
          +----------------------------+-----+------+

 

NPN + NPN:

               /
          +---+ +---|>|-------+---------------------+
          | SW1    10A10      |                     |
          |                   \              TIP41C |
          |                   / R1           2N3055 |
          |                   \ 100 Ohm       kt819 |
          |                   /                     |
          |                   |                   |/ Collector
          |                   +-------------+-----|
          |                   |             | Base|\
          |                   |Collector    |       |Emitter
          | 2 cells            \            |       |
          | Li-Ion       2n2222 \|Base      |       |
          | 18650                |----------)-------+
          |                     /|          |       |
        -----                  /Emitter R2  \       \ R3
         ===                  |        10k  /       / 
          |                   |             \       \
          |                   |             /       /
        -----                 |             |       |
         ===                  +-------------+-------+
          |                                         |
          |                            +-----+------+ 
          |                            |  C1 |      | 
          |                        R4  \  10u|      | Laser
          |                        10k /    ---    _+_
          |                            \    ---    \ /
          |                            /     |     -+-
          |                            |     |      |
          +----------------------------+-----+------+

 

One could imagine a MOSFET+MOSFET here, but due to (comparatively) high voltage needed to turn a MOSFET on the voltage drop over the current-measuring resistor R3 would be 2.5-3 volts even for the so called "logical driven" MOSFETs. And it is quite an overhead.

 

In the NPN+NPN variant the circuit was tested with a 5W 808 nm diode, The kt819 transistor appeared to be tough enough, so three channels in parallel were used. A "heavy-caliber" laser pointer worked flawlessly until the attempt to use a commercial constant current driver there. That diode was burned out, but the described driver has nothing to do with that.

 

Another variant was specially intended for use with batteries:

              /
          +--+  +--+--|>|-----+---------------------+-----+
          | SW1    |  10A10   |                     |     |
          |        |          +                C3   |     |   5W
          |       --- C1       / SW2          3.3n ---   _+_  808 nm
          |       --- >100uF  +                    ---   \ /  laser
          |        |          |                     |    -+-
        -----      |          \                     |     |
         ===       |          /                     +-----+
          |18650   |          \ R1                  |
          |2 cells |          / 100 Ohm     irl540  |
          |Li-Ion  |          |                   |-+Drain
        -----      |   +------+              Gate||
         ===       |   |      |__________________J|-+
          |        |   |      |             |     |-+Source
          |        |   |      |Collector    |       |
          |        |   |       \            |       |
          |        |   | 2n2222 \|Base      |       |
          |        |   |        /|----------)-------+
          |        |   |       /            |       |
          |        |  ---     |Emitter  R2  \       \ R3
          |        |  --- C2  |        10k  /       / 0.14 Ohm
          |        |   | 0.2  |             \       \
          |        |   | uF   |             /       /
          |        |   |      |             |       |
          +--------+---+------+-------------+-------+

driver

drv_mosfet1a drv_mosfet1b

 

In previous variants the diode was connected in series with the current regulator. It is correct, since the constant current generator makes the current to be constant in anything connected in series with it. As one can see in the last scheme the diode is included into the drain circuit of the mOSFET. This allows to avoid the subtraction of the voltage drop on the diode from the total power supply voltage. So the full supply voltage is available to control the gate of the MOSFET.

 

The drawback of this connection is that the laser diode is under a voltage in relation to the common wire (to the ground wire). Sudden shortcircuit to the ground - and the diode is dead. Nevertheless in battery powered circuits the term "ground" is rather conditional one, and we can utilize such a connection if not planning to feed the device from the mains.

 

If one uses a logic level MOSFET (like irl540 or irlr2905), if the laser diode is red or infrared and if the reverse polarity protection diode is excluded (shorted), the scheme becomes able to operate from a single Li-Ion battery (3.7V). It is cheaper, occupies less size and also there is much less of heat on the transistor. The price is however is the danger to kill the diode by inserting the battery in reverse.

 

In all variants of the circuit there is a sense to compose the R3 resistor from several discrete ones. The necessary value of the constant current can be selected by soldering the necessary amount of the resistors. It is clear that until the proper current value has been set, one should never connect a real diode as the load. Use a 0.5-1 Ohm resistor there or a pair of powerful rectifier diodes in series there.

 

C1 capacitor is the protection from the SW1 switch ringing. The ringing of SW2 affects performance much less, because SW2 conducts only a small current, limited by R1. Also the soft turning on (provided by slow charging of C2 through R1) is the very mean to lessen the harmful influence of the switch ringing.

 

THE MALUCE OF THE CIRCUIT is there is a lack of reference voltage. In practice it means that any newly assembled circuit needs individual tuning by variation of R3 value. There is no compensation of the temperature drift also. Besides for the independent reasons You don't want to turn on this driver when it is too cold (winter) or too hot. In the first case there is a danger to kill the diode due to the humidity condensation. In the second case there is a danger to kill the power MOSFET and/or the laser diode due to the overheat, since the circuit has no means of the overheat protection.

 

THE BONUSES OF THE CIRCUIT are:

  • The possibility to operate using a single Li-Ion battery (when using some logic level MOSFET and without the reverse polarity protection diode);
  • Excellent damping of transient current under the dynamic load variations due to the absence of a large capacitor in parallel to the laser diode.

pointer01 pointer02

 


 

Both circuits are simple enough and if there are no soldering errors they begin to operate properly right after having been assembled. The only tuning is to chose the correct value of the current measuring / current setting resistor (R2-R5 in the circuit with on-chip voltage regulator and R3 in the circuit based on two transistors). In order to do it, solder a dummy load to the output terminals of the circuit (in place of the laser diode) and adjust the value of the current setting resistor until the desired current is obtained. In the simplest case one can use a resistor (0.5-1.0 Ohm rated) as the dummy load.

 

The current through the load may be measured in several different ways:

  • If a simple resistor is used as the load one can simply measure the voltage drop on it using some multimeter, The value of the current can be readily obtained by applying the Ohm's law.
    driver_control
  • One may connect a multimeter in series with the load. The multimeter should be switched to ammeter mode in this case. For these measurements I advise to make an additional set of probes for the multimeters. The proper probes should be short enough (not longer than 10 cm) and made of thick wire.
    driver_control2
  • One can attach an additional current-measuring resistor in series with the load and measure the voltage drop on it. This method is especially useful when measurements are made by an oscilloscope.
    driver_control3
  • And finally one can measure the voltage drop over the current setting resistor. However two conditions are to be met here:
    First: the value of the current setting resistor must be known precisely. It is easy if the one is formed by a number of commercial resistors. And it is almost impossible if You are using a piece of wire there and adjusting the length of the wire until the necessary load current is obtained. Second: the stay currents there are to be negligible in comparison with the main load current.
    The stray currents are: the ADJ lead current in the circuit with on-chip voltage regulator or the base current of the control transistor in the circuit based on two transistors. This is easy if the diode to be used is powerful and the full load current has the value of several Amps, and this is not so easy when trying to drive diodes of low or medium power.

 

Sometimes the two-transistors driver shows a tendency to oscillate at high frequency (like ~30 MHz). Sometimes it takes place only in moments of switching on/off. In other cases it can show permanent oscillations. In case of permanent oscillations it is easy to diagnose them with a multimeter. If the circuit gives too low current (in comparison with the expected value) and if there are no soldering errors, it means the HF excitation. The temporary HF bursts during switching on and of can only be seen using an oscilloscope.

If the HF excitation takes place one can try to jam it by including some 50 Ohm resistor between the base of the control transistor (2n2222) and the point of signal taking from the current measuring resistor R3. One may also use ferrite beads, since the main purpose of their design is to dampen the HF oscillations. One can also try to replace the control transistor (2n2222) with another one, having lower frequency border. The russian kt3102 works good here. I am sorry, but due to the lack of testing, I can not recommend some good transistor from the international nomenclature.

And finally the most correct and effective way to jam the RF oscillations is
to reorder the printed circuit board geometry.

 

When You have tuned the scheme, You need to check whether it really does regulate the current or not. In order to do this measure the current several times, each time attaching different resistor as a load. The current should stay reasonably constant. (For the two-transistor circuit the changes of current in range of 5-10% are acceptable.)

One need to keep in mind that both schemes require that the voltage drop over the load should be less than the supply voltage minus the voltage drop over the current-setting resistor and minus some additional voltage drop of the regulator element (~1V for the on-chip regulator and bipolar transistors and equal to the voltage drop over the open channel resistance for MOSFETs) In other words You can not choose too high value of the load resistor.

On the contrary You can freely choose the value of the load resistor to be as low as zero. Since the circuit regulates the current it wont affect much its safety or performance. Nevertheless If You measure the current by the voltage drop over the load resistor get ready to face huge measurement errors id using too small resistance there.

 

If You have an oscilloscope (and if You are going to make some laser diode drivers You definitely ought to get one) then BEFORE ATTACHING THE REAL LASER DIODE TO THE OUTPUT OF THE SCHEME check the absence of current overshots when switching the circuit on and off. For the two-transistor scheme it is good to check for the absence of current overshots during dynamic load changes. To do this make the dummy load using the next schematics:

 o--------------------+-----+
                      |     |
                      |     +
                      |      / SW
                      |     +
                      |     |
                      /     /
                      \R1   \ R2
                      /     /
                      |     |
 o----+---/\/\/---+---+-----+
      |     R3    |
      ^           ^
      A           B

Where R1 and R2 have the value of the order of the laser diode resistance at its working mode. (For example for a five watt rated infrared diode: R1 = R2 = 2.4V / 5A = 0.48 Ohm ). The R3 resistor should have its value as large as R1/10...R1/5.

The oscilloscope's probe is to be connected in parallel to the R3 (points A and B on the scheme). When turning the SW switch on and off there should be no noticeable overshots and ringing.

 

Generally the powerful laser diodes are the devices heavily overstressed in several parameters (Like the electrical and optical power density, electric field tension and so on) so they die by every sneeze. And they cost damned high and to loose one is a tragedy and funeral. So to use the laser diodes is less science than a mysticism and religion. The science is based on observational facts and argumentation. The religion is based on dogmata.

 

DOGMA THE FIRST - DOGMA OF LASER DIODE INFALLIBILITY.

It can be expressed as that: IF THE LASER DIODE HAS DIED ITS POWE SUPPLY IS GUILTY.

There are other dogmata too, but lets stop here for a time.

 

It is considered that even the smallest surpass over the allowed limits, even for the shortest time is strictly forbidden. For example a datasheet of 5 watt rated infrared diode says that the maximum current is 5 amps. One should get is "YOU are not allowed to overshot it even by 1 mA even for 1 ns" Moreover it is considered that the reverse polarity is strictly forbidden. Even for 1 mV over 1 ns. It means You can kill the laser diode by Your multimeter in an attempt to find its correct polarity. (In practice I don't remember any case of killing a laser diode by a multimeter, but it is HERESY to doubt in dogma)

Even worse that it is considered that one must never exceed the maximum allowed output power. (Even by one milliwatt during one nanosecond). Why I say "worse"? Because in general the watt-ampere characteristic of the diode (in other words its efficiency) depends at least to its temperature. And You can easily exceed the top allowed power even when keeping the supply current in the allowed limits. Just by using it outdoors in cold weather. It is considered that a proper driver should contain an optical feedback to keep the output power in its limits. For the further details I'd better send You to Sam'S Laser FAQ, because we wont apply all those features here. Otherwise our simple LD driver will blossom with hundreds of protections and thousands of sensors, and it will grow a data center nearby to process the data from all the censors and to elaborate a strategically weighted solution on its base.

 

Dogma of the diode infallibility leads to two things in our case:

First: If Your don't want to shed the tears on the prematurely passed away laser diodes (our precious), there is no such words as "forgot" or "was lazy" to You:
"Forgot to check the current"
"Forgot to check for overshots"
"Too lazy to check, I've build a thousands of such drivers, what can go wrong?"
Second: When You know that the smallest excess of parameters is strictly forbidden, and when You know that any real device (including Your driver) has some spread of parameters, the obvious conclusion is that You must "derate" (or "downrate") the device. It means You must provide a sufficient difference between the value of the current You driver supplies and the value of the top allowed current. That safety margin must be large enough to keep the current, power and any other parameters in their safe limits in any case. (In case of any variations of the supply voltage, of the ambient temperature, etc.)


Performing deratization, sorry, derating of the diode You are being between two sides: on one hand You don't want to put too much power to avoid killing the diode and on the other hand You don't want to pot too low power - or else would not it be simpler to buy a 10 mW green pointer in the nearest stall? It is understandable that if one does not exceed a half of the top allowed current (e.g. 2.5 amps for that 5W rated infrared diode, having been used as example many times above) it almost warrants the safety of the diode, but the output power won't be acceptable.

Blessed the one, having a bunch of laser diodes, and having the opportunity to kill a few of them to estimate the borders of the allowed. Others are usually in lack of such possibilities.

Sometimes the manufacturer suggests the optimal (to his opinion) amount of derating. For example Nichia NUBM08 diodes are strongly recommended to be fed by a current not exceeding 3 amps. At the same time the table of absolute maximum rating in its datasheet contains the value of 3.5 amps. Most other manufacturers lack such a respect to the clients.

 

The real value of the current You decide to set depends:

  • To Your assurance in the driver's reliability.
    The lower is the precision of the current regulation, the larger safety margin You will choose.
  • To Your assurance in the laser diode reliability.
    The diodes of top rated manufacturers (such as nLight, Osram) have a good probability to operate flawlessly at the current of 95-99% of the top allowed one. "Noname" diodes should be fed by the current that corresponds
    to 60%-80% of the top allowed output power. (If You have no power meter - set the current to 60%-80% of the top allowed current for this diode.)
  • To Your prosperity.
    Indeed if You can bear easily the death of a few laser diodes, You can drive the diode to the maximal allowed current (or even higher) without the excess of care. And for You it may be worth to burn out every other laser diode only to boast the "most powerful laser pen in the world"
  • And finally it depends to Your bravery/insolence.
    If You feel being the gambler, I recommend to set the current to 110%-120% of its maximum allowed limit. In case of success You will become the owner of the "most powerful in the world, etc, et..." In case of fail - just consider it wasn't Your day. In any case I would not recommend to exceed 150% of the maximum allowed limit even to the most hardcore gamblers.

 

PWM TYPE POWER SUPPLIES

The analog current regulator requires the supply voltage certainly higher than the expected voltage drop over the diode at its working mode. And also the analog driver has rather low efficiency.

There exist a solution, free from those shortcomings. It is PWM type power supply. However there are no free bonuses in the world.

1st: The pulsed mode power supply has much more points and reasons to fail than its analog relative. And each fail can cost a life of the precious laser diode;

2nd: Working pulsed mode power supply is at the same time the powerful source of electromagnetic interferences;

3rd: If You are using a commercial device, You do believe that You money is the warranty that all the problems were solved successfully for You. And it is not always true.

 

Occasionally this part will be filled with the schemes of PWM power supplies and their description. For the time I'll say only that at the present cost of the laser diodes it is scary even to connect them to the analog power supply. And the hands are shaking already from the thought about to connect the diode to the pulsed mode one.

 

THE LASER DIODES

When choosing/buying a laser diode one should pay attention to:

  • Power
  • Brightness
  • Color (wavelength)
  • Lifetime

 

Power is the main parameter of the laser diode if You are going to cut or drill something with the diode. E.g. to cut plywood on a CNC machine. A diode becomes reasonably suitable for usage in CNC machining beginning from 5 W of power (well at the bare least - from 3 watts of power).

 

For the usage as a laser pointer the factor of the most importance is the brightness. The visual perception of brightness is also affected by color. The perceived brightness of the spot of green laser pointer (532 nm) is almost 10 times higher than the perceived brightness of blue (450 nm) or red (650 nm) laser pointers, and this is when the output power (in watts) is equal.

If we drop away those twisted photometric values, then the brightness will be just the output power divided by the (3d) divergence angle and by the area of the light emitting surface. And in this sense the brightness does almost directly determine the range of the laser. It means not only the ability to point to a monument 5 miles away from You, but also the transmission range of You photo-phone using the diode as its transmitter, and the ability to light a match at the distance of 15-30 feet.

If You need range - choose brightness even to the detriment of power.

The brightness is kinda reverse value to the entropy. One can not increase it without the work of the applied forces. On the other hand there are much and many opportunities to decrease it down.

In order to determine the brightness You need to know the power of the diode, the divergency of its beam and finally the sizes of its emitting surface. Here are the internals of the common diode without its top cover:

 

laser_diode

 

The emitting surface is characterized by its sizes h and d. The height h is the thickness of the pn-junction. And one can estimate it as being almost equal to the working wavelength. The width of the emitting surface is sometimes not equal to the width of the semiconductor crystal (as it shown on the picture). Usually one can get this value from the datasheet, If there's no datasheet treat it as if it was equal to the width of the crystal. If You have a possibility to turn on the diode and You have a lens with You, the next simple experiment will allow You to discover the width of the emitting area:

 

experiment

 

Focus the radiation of the laser diode onto a screen (onto a wall in the simplest case). Better not to use the full power to avoid burning of anything. Try to achieve as sharp image as possible. Then measure the distance l between the diode and the lens, and the distance L between the lens and the screen. Also measure the size of the strip of light on the screen D. Finally You can get the size of the emitting area d from the following obvious proportion: D/d = L/l .

 

Concerning the divergency, one should keep in mind that laser diode has two very different directions of divergency. If taken without any (internal or external) optics the laser diode will show its spot in a shape of an
elongated rectangle:

 

axes

 

Along the axis orthogonal to the plane of the crystal (to the plane of the pn-junction) the angle of divergency O_|_ is large. Usually it is 60 or even 90 degrees. Thus this axis is named "fast axis" (the axis along which the beam diverges fast), and the angle itself is named "angle of divergency along the fast axis".

Along the axis parallel to the plane of the crystal the beam diverges slowly. 8 to 12 degrees usually. So the axis itself is called "slow axis".

The fact, that the divergency along the slow axis is lower than along the fast one, does not mean that the "beam quality" is better in this direction. If we consider 2D problem in the plane, that contains the fast axis, we can understand, that in this 2D problem the laser diode can be treated as practically ideal pint-like source of light. And thus the radiation can be shaped into an almost ideal parallel beam, limited only by diffraction and aberrations of the lens.

Another situation if we consider 2D problem in the plane, that contains slow axis. Now the source has certain (and not negligible) size. And when You will try to focus the beam onto a far away screen, You always will deal with the image of the source (as it was on the picture on the determining the size of the emitting surface). One just can not make the spot to be smaller than that image.

 

That "foolish" beam shape makes the life worse if one needs to focus or to collimate the beam. (Collimating is the process of making the beam to be as parallel as possible. Usually it is done by a lens, by installing the emitting area of the laser diode into one of the foci of this lens. The lens, being used in collimating purposes is known as "collimating lens" or simply as "collimator".) To perform this task one generally needs an astigmatic optics - cylindrical lenses or anamorphotic prisms (see further in the OPTICS part) however those parts are less affordable and cost higher than the common (spherical) optic parts.

 

To make the life easier for the ones, who does not want to deal with the astigmatic optics, the manufacturers produce diodes equipped with internal cylindrical lens, that partially compensates the fast axis divergency. That lens is called "Fast Axis Compensating lens" or simply FAC-lens.

 

axes_FAC

 

Don't think that FAC lens is able to increase the brightness of a laser diode. The decreasement of the fast axis divergency costs the serious drop of the beam quality. In most cases You can treat it as if the diode with FAC lens has the enlightened area of the FAC lens as its emitting surface. And it is much larger than the pn-junction area.

Consider the example. Lets assume that the initial diode without any lens has the size of its emitting surface of 20x200 mcm and the power of 5 wt. Then using a lens with 100 mm focal length You will be able to focus its beam into 1x10 mm sized spot at a distance of 5 meters. It corresponds to 50 W/square cm intensity, which is enough to puncture a paper, to light a match and to make many other useful things.

If the same diode was equipped with FAC-lens, having a shape of a cylinder 100 mcm in diameter (this can be often seen in practice), You can assume that the emitting area now has the size of 100 x 220 mcm. (The last number has increased due to the aberrations of the cylindrical lens.) If You again use a lens with 100 mm focal length, You will be able to obtain only 5x11 mm spot at the same 5 meters from the diode as it was in the first case. The new intensity will be only 9 watts per square centimeter. I.e. five times worse than it was in the paragraph above.

However in the first case You need a pretty large lens, moreover the one having its diameter being equal or more than its focal length. (The minimal diameter of the lens should be taken from the condition, that the lens should capture all the light emitted from the diode, and, hereby, is determined by
the fast axis divergency.) On the contrary in the second case You need a lens with 100 mm focal length and 20-30 mm diameter. Such a lens is cheap and affordable.

I hope, that this example has shown that it is hard to say, whether to choose the FAC lens or not, without knowing the details of the application and without taking into account ones possibilities to assemble the proper optics. Generally, if You need to cut and drill something in 3-5 cm in front of Your diode, it is more wise to choose diode without FAC lens and rely on the abilities of good spherical optics. Otherwise, if one needs high range, and if the possibilities of using the astigmatic optics are limited, one should better choose diode with FAC lens installed.

 

Color (wavelength) affects not only the perceived brightness of the laser spot. The absorptive ability of the material depend to the color strongly. The amount of absorption means the amount of energy available to cut or drill the material and therefore determines how easily or hard it can be done. A classic example is white paper. One can easily puncture it by a blue laser pointer. Not so easy it can be done by a green one. And with a red one it is very very difficult.

Nevertheless from the point of view of materials processing the color is still secondary factor after the power. It is because organic materials can easily be charred, and after that they absorb almost every wavelength. Metals may be painted and/or annealed to increase their absorption. And glass... if it is colorless, it makes no difference between red or blue - any visible lasers are useless to process it.

 

From that can be observed now on eb@y and aliexpre$$ one can come to the conclusion that there are the next kinds of diodes:

  • Invisible infrared type (920-980 nm) - their power is up to 10 W and they cost like ~30 USD/W
  • Visible infrared ones (700-820 nm) - their power is up to 10 W and they cost like ~30 USD/W
  • Red ones (of different color in range 630-670 nm) - with power up to 1 W and cost ~ 40 USD/W
  • Green ones (520 nm) - with power up to 1 W and cost ~ 80..90 USD/W
  • Blue diodes (440 - 460 nm) - with power up to 6 W and cost < 18 USD/W may cost <9 $/W if in stacks like NUBM06-NUBM08
  • Violet ones (405 nm) - with power up to 1 Wt and cost like 35 USD/W

 

It is understandable, why green diodes cost so much - they appeared recently and there is still deficiency of them. Less understandable why red diodes still cost high and have rather low power despite the fact that they were the first to appear at the market.

 

Infrared and red diodes are made of Gallium Arsenide (GaAs). Actually there is a puff-pastry inside them. It is so called "heterojunction". However it has nothing to do with DIY skills, so it is simplier to consider that these diodes are based on GaAs only. Green and blue diodes are made of Gallium nitride: GaN.

What does this knowledge mean for DIYer? For example one can try to tie the known special properties of the diodes with the material they are based on.

It was said above that all the Gallium Arsenide based diodes, I had, did operate well when fed by a current regulator, based upon on-chip voltage regulators, like LT1083 and similar. There was no exclusion. It means that their current-voltage curve is of non-decreasing type. On the other hand the fails with those drivers took place and blue diodes. If it is due to the properties of the semiconductor, one can suppose that all nitride diodes (blue, violet and green) have the property of dynamic variation of their resistance. And thereby they cannot be fed by any kind of driver having a large capacity at its output.

Another observation - different temperature/overheat endurance of the diodes of different types. I personally have sent a few arsenide diodes to the heaven by powering them without a proper heat sink. It even happened that these diodes went to their ancestors just after overheating in the process of soldering. Nitride diodes haven't yet been noted in such a behaviour. Nichia, for example, allows operation of their diodes at the temperatures up to 70°C, and storage at the temperatures up to 85°C.

The higher heat durability of the nitride diodes is not necessary directly related to the semiconductor properties. It may, for example, be determined by the types of the solder alloys, having been used to assemble the diode. However the type of solder alloy is selected with taking into account the properties of the semiconductor. The arsenide diode contain one type of solders, while the nitride ones contain another.

 

The importance of laser diode lifetime is understandable for the use in some CNC machine. It is not so evident for some laser pointer, where the total working time may be less than a hour during the whole time of its existence. And here one should remember that along with the operating time there exists a parameter describing the lifetime "on the shelf". Every manufacturers does proudly present the operating time of their diode. But the on shelf lifetime is usually being glossed over modestly. And what a pity, when three years later, one takes three watt rated "her brilliance" from the shelf and finds out that it can be addressed only as "mrs. Grey."

Here is an example: two my (not homemade) laser pointers. They aren't from wicked lasers actually, so don't expect much. I will call them as "the blue one watt rated one" and "the blue two watts rated one", since their manufacturer and distributor aren't known well.

 

Measurement results:
  July 2015 January 2018
"the blue one watt rated one" 650 mW 576 mW
"the blue two watts rated one" 1160 mW 710 mW

 

The work hours on any of these pointers is in any case less than 10 hours (and this is an upper estimation with a pretty large margin). The batteries in both cases were new and freshly charged. The current through the diode was not measures though, but it is assumed that the driver can support it on the same and constant level.

 

I have no such a nice data on the infrared diodes. Two 5-Wt rated diodes were bought simultaneously in 2014. The first of them was used and finally burned out, the second one had stayed sealed until the very recent time. However I can present two numbers: the diode, that was used in 2014, did 3 W at 3.5 Amps, and the other one does 3.2 W at 4.7 Amps now. If we assume that both diodes were equivalent at the beginning, and if we let the threshold current be 0.5 Amps, we can conclude that in 2014 they had 1 Wt per Amp, and now they have only 0.76 Wt per Amp. Not a very precise or reliable data, but still, the wind blows into the same side as with the laser pointers.

 

Why should I think that in the observed power reduction the laser diodes are guilty? One could say that the optics has became dirty (no, it didn't, it was checked out), or the calibration factor of the laser power meter has drifted (no, it didn't - checked also) or either the supply current having been measured that time does not correspond the present one (in both cases it was measured by new Chinese multimeters.... Maybe the Chinese have changed the Amp definition?) Also one can say that it is due the temperature, ambient pressure, humidity, season of the year, lunar phase....

 

DOGMA THE SECOND - PRESUMPTION OF DEGRADATION.

What does it mean, degradation? It was already said about the ability of laser diodes to fail due to every sneeze. And the degradation is the ability to die step by step EVEN WITHOUT ANY SNEEZE. I.e. during the normal operation or even during storage on the shelf.

It is considered that the ability to degrade is the main property of laser diodes. Well... Maybe not exactly the main, but at least the second one after the ability to emit the light. I hope I don't need to say that I'm kidding slightly. But every joke contains a bit of truth...

The mechanism of the process is still incomprehensible to me. More common to deal with elements that have a certain and constant probability of fail. Like transistors or diodes. If, for example, a new transistor has a probability of fail ~10% per 3000 hours of work, You can be sure that a used transistor has the same probability of fail. Exactly for the same reason as the probability of flipping coin to show the heads is fully independent to how much time it has shown the heads in the previous flips.

On the other hand the finite lifetime of a laser diode presumes that the diode contains something expendable or something able to wear out. In order of nonsense I can suppose that the semiconductor has a non-zero ionic conductivity. Too small to observe it on the background of the large electron-hole one, but still there is. As there is an ionic conductivity, consequently there is an electrolysis. So there are metal dendrites growing from the leads into the semiconductor volume. Initially they diminish the transparency and finally cause the complete fail. As I've said it is nonsense, but it is still better than to consider that there is a finite reserve of electrons and vacancies or that all the stored inside photons can fly away.

 

OPTICS

Hard to imagine the practical application, where the natural ray of diode laser would suit. Maybe the usage as a flashlight or, maybe, for holography. (However the low coherence of the radiation of laser diodes makes the latter a bit difficult.)

In all other cases one either needs the laser spot to have minimal possible size in order to have the highest possible intensity, or either needs a parallel beam. In order to shape the radiation into the parallel beam, or in order to focus it, one needs a lens, i.e. optics.

And the very first property to know is that ANY OPTICS BRINGS ITS LOSSES. Professionals, having access to the fancy lenses with high-tech coatings, got used to bounce the beam back and forward as many times as they wish. But DIYer is usually free of this opportunity. And the reason is not in the rarity of the lenses. The reason is exactly in these additional losses.

The reflection on the border of two media, having different refraction indices (n1 and n2), when the ray goes at the rectangular angle to the border, can be expressed with the well known Fresnel formula:

                (n2 - n1)^2
           R = -------------
                (n2 + n1)^2

For example each border between air (n1=1) and glass (n2=1,5) reflects 4%. And it is the best case, when the surfaces are extremely clean and even. Any dirt or roughness makes the things worse. One can think that the reflected light is not lost yet. It seems like it may be useful, for example, the reflected surface is well aligned. However this is wrong for many reasons. E.g. lens has a spherical surface and gives divergent (or convergent) reflected beam. And when we try to focus the radiation its focal spot is far away from the focal spot of the main beam. Another reason is that it is a bad idea to let the reflected beam to enter the laser diode. So we just can not align the surfaces in that way... So any reflection means nothing but losses. And in its own turn it means that any lens, any prism or glass plate reduce the power by 8..10%.

Take for example the 5 W diode. After it has been equipped by a collimating lens it gives only 4.6W. A two-element objective with a protective glass will cut the power to 3.9 W. And the full scale optical system with the astigmatism compensation, consisting of two cylindrical lenses, two spherical lenses and certainly with the protective glass will leave only 3.3 W alive.

Are the things really so bad? At least there is an antireflection coating... Yes it is there. But not here. The wavelengths of the most powerful laser diodes are either outside of the "visible" range or at its borders. And these borders are not taken into account by developers of the common optics, that can be found in binoculars, cameras, etc... Occasionally the exclusion from this rule will be the green diodes. But they will only be interesting when they reach several watts of power and when their price will quit to cause a hysterical laugh. And if one uses some optics with antireflection coating intended for other wavelength (or with decoration coating) it will only cause even more losses. If there was something like 4% per each border of media, it can easily reach 10% or more in this case, and even the sole collimator lens can make Your 5 W rated diode to 4W one. The full scale five-element optical system will humiliate if to the state of 1.5 W.

 

Geometrical addition of beams

The divergency of the beam of a separate laser diode is often suitable for most application. Remember laser pointers, for example. However the power of single diode is often less than acceptable. The straightforward way to increase the power is to take several laser pointer and wrap them tightly into one bunch with some duct tape. If those laser pointers are good and all the rays from this bunch are going almost parallel to each other, then it takes only to cover the resulting beam with a lens and all the rays will have their focal spots close to one another. Like it was done there.

Its a pity but this approach has several failures. To be exact the sole failure is only the low brightness when being averaged over the whole bunch of rays. But it is simpler to explain on several examples:

 

First: If You need to focus the beam, it will eventually occur that the intensity is still too low for You. E.g. laser saws wood successfully, but You want to saw steel. You will need to make a smaller spot and it will take You to have a lens with shorter focal lens. But we know, that a lens with short focal lens just can not have a large diameter, So by taking smaller and smaller lens with shorter and shorter focal length You will eventually loose some rays, as at the picture below:

many_diodes_focusing

 

Another example is drilling. If the beam. If the beam converges rapidly after the lens and then diverges rapidly after its narrowest place (after its neck), You won't be able to drill a through hole in a thick material. And in reverse: if the beam is narrow and converges slowly, it has a long neck, that allows to drill and cut thick materials, as it is shown on the picture below:

beam_neck

 

The third example. Imagine, You want to send the laser beam far far away and to have the spot there as small as possible. In this situation You definitely are to expand the beam by some telescope. (Remember, how a telescope work: "if the beam diameter was increased by k times, its divergency becomes k times lower".) But if the beam is already wide, it is hard to make it even wider. If we take, say, a beam, having 5 cm diameter (like here) and if we try to expand it, say, 5 times, we will end up with 250 mm lens. And it is comparatively large one, almost like professional astronomy instrument. It costs high too.

 

The said, i think, is enough to understand that it is bad to have a large spacing between the separate rays of the separate diodes. But even if we placed the diodes into close connection with one another we will still have low effect - the bodies of the diodes, the lens mounts and etc will become the obstacle to get a most compact beam. The ideal case would be if we could place the rays "one inside another", without occupying any additional place. However it is not always possible to do so. The partial solution of the problem is given by geometrical addition, where the rays are placed not "one inside another", but "one as close to another as possible".

One of the possible schemes of the geometrical addition is shown on the next picture:

knife_edge_mirrors

 

The mirrors are intentionally shown as being bigger than the rays sections. One should note that it is not necessary to have too little mirrors in order to construct this scheme.

The picture makes clear, that if the edge of each of the mirrors is straight, and if there is no gap between the edge and the end of reflective coating, the spacing between the separate rays can be made very small. The united beam will be almost continuous.

It is clear that for the united beam have small divergency, the separate rays must be as parallel to each other as possible. So the mirrors are to be installed precisely. Worse than that, if the position and direction of laser diodes has some scatter, it means that the installation angle must be individual for each mirrors. In its own turn it means that the mirrors have to be alignable, or there have to be a possibility to align them during the installation (gluing up).

Fortunately it is not too hard to provide this. One can use a simple tool. Its photo and cinematic scheme are shown on the pictures below:

alignment_tool

cinematic_scheme

 

It is robust to use a match as a mirror keeping handle. After the mirror have been glued up, one can easily cut the keeping handle by some laser. This way You can avoid mechanical stress, that can ruin the alignment. Don't use bamboo sticks from mats. Unlike matches they are not paraffined and use to bend and twist when the ambient temperature and humidity change, Generally the technology it the next:

  1. First of all glue up the first mirror. Its up to You whether to use the special tool or not at this stage. It does only matter that the ray was directed in some proper direction and did not touch anything on its way. Only the first diode must be turned on at this stage.
  2. Give the glue a time to cure completely. If You are using the tool, cut off the mirror keeping handle.
  3. Take a new mirror and glue it to a new keeping handle (to a match) and install the keeping handle into the tool.
  4. Now You need to set the mirror into proper position. Since the boom of the tool is not movable, the only way to do this is to move the platform where Your diodes are installed (the heatsink with the diodes, if You want)on the tool basement under the boom. Find the position, when the ray of the second diode hits near the end of the second mirror as it was shown on a picture above (the enumeration of the diodes and mirrors is from left to right). Keep an eye for the (second) mirror not to become an obstacle for the first beam. The fist and second diodes are to be turned on now. It is also good to keep the angle between the first and the second rays in reasonable limits, or else the alignment range of the tool may be just not enough to align the rays.
    When the proper position of the platform with the diodes have been found, glue it up to the basement of the tool with several drops of a hot glue (aka glue-gun or low molecular weight polyethylene). This attachment is to be removed later, so don't be too zealous when gluing.
  5. Align the ray of the second diode to the ray of the first one. Add some glue to fixate the second mirror and leave it aside to cure up. Dependently to the type of the glue it may take from several hours to several days. I can not recommend to use the glues, that take more than one day for the complete curing. Otherwise the weather (the ambient pressure, temperature and humidity) may drift far enough to cause problems with the alignment, even if the work is being made indoors.
  6. When the glue has cured, check the correctness of the alignment (by turning on the first and the second diodes). Then cut off the mirror keeping handle. It is better to cut without applying any mechanical stress - for example by means of some laser pointer, rated to 1-2 W. After the keeping handle have been cut, it has a sense to check again if the alignment is still proper. If yes - proceed to the further step. If not - it is a good time to remove the mirror (while the strength of the glue has not built up) and return to the step 3. The first mirror may be kept untouched.
  7. Detach the platform with the diodes from the tool's basement. (Obviously the position of the platform relatively to the boom of the tool in any case must be changed for the next mirror installation. Such a simple tool allows to install only one mirror at a time.)
  8. Take new mirror and attach it to new keeping handle. Install the keeping handle to the tool.
  9. As You did it in step 4, find a position, when the ray of the third diode hits mirror near its edge, and when the mirror does not make an obstacle to the pass of the beam of the second diode. The second and the third diodes are to be turned on here. When the proper position was found, glue the platform with the diodes to the basement of the tool.
  10. Attention here! At the fifth step You were aligning the ray of the second diode to the ray of the first diode. You may propose, that now You are to align the ray of the third diode to the ray of the first one. It is not correct. Due to the alignment errors it is better to align the ray of the third diode (and all the next diodes) to the ray of the first one. Evidently the first and third diodes are to be turned on now.
    Fix the position and rotation of the third mirror by the glue and let it to cure. One may (and should) turn the diodes off for the drying period of time.
  11. When the glue has cured completely, check the alignment (by turning on the first and the third diode). Cut off the mirror keeping handle and check the alignment again. If the alignment is improper, detach the third mirror and repeat its installation from the very beginning (from the step 7). One may leave the first and second mirrors untouched.
  12. If You are satisfied with the alignment, repeat the procedure for all the following diodes and mirrors. You should already be familiar with it.

 

The result can be seen on the following photos and videos:

  • The photo of a half of NUBM08 diode bank having been collimated by mirrors
    and installed onto a heatsink.
    NUBM08_with_mirrors_on_heatsink01 NUBM08_with_mirrors_on_heatsink02 NUBM08_with_mirrors_on_heatsink03
  • The photo of a laser beam spot in the far field (at a distance of 5 meters):
    NUBM08_farfield_spot
  • The video of the laser beam cutting wood and metal:

    With using a 50 mm focal length lens I succeeded to drill a hole in a razor blade and the dimensions of the hole were: 0.7 mm x 0.2 mm. The whole beam comes through this hole, it can be detected by the fact that the razor blade quits to heat up after the hole is complete. So the focal spot has its sizes less then those 0.7 mm x 0.2 mm. When the output power is 15 W it gives intensity I = 15 W / (0.07 cm x 0.02 cm) = 10.7 kW/sq.cm.

 

Several notes at the end of this part:

  • Epoxy resins are robust glues for gluing up the mirrors. There exist resins that can cure in 3-5 minutes. However in reality this is not curing but rather "capture". Actually the speed of the real curing for the most of the "fast" epoxy resins is lower than the one for the normal "slow" resins.
    Additionally it is hard to apply a photo curing resin here, since it tends to cure up under the radiation of the diode being aligned. Before use a resin from a given tube, it is recommended to make a test mix - to see how long it will take to cure, and how hard it will become at the end. The experience shows, that the scatter of these parameters from tube to tube may vary more than from one type of epoxy to another.
  • The final result of the addition of the rays may be better if one makes the "anticipating" alignment - with taking into account the predicted drift of the alignment for the time of the epoxy curing. The drift of
    the alignment is usually not due to the shrinkage of the resin (that is the first one thinks about), but due to the instability of the alignment tool itself. Having got some experience You will know the direction and amount of the alignment drift dependently to the time of drift and to the history - to the magnitude and direction of motions, having been done during the alignment. With this knowledge You will be able to find such position and rotation of the mirror, that will end up to the proper position and rotation to the moment of glue curing.
  • The alignment is better to be made not at the full power of the diodes but at some smaller one. It will provide more safety to the diodes and prevent overheat of mirrors, which is detrimental when the glue has not yet build up its hardness.
  • Note that the description above is a framework only, for not to sink in details like each switch position and each motion. Particularly it's up to You to decide what diodes are to be turned on and of and when. Remember only that You should wear protective glasses even if only one diode is powered up and even if the power has been reduced.
  • If the fast axis is orthogonal to the plane of the platform containing the diodes there appears an interesting case, see the picture below:
    near_and_far_field1
    (The fast axis is orthogonal to the plane of the picture.)
    The ray of a laser diode diverges rapidly along the fast axis until it hits the correspondent lens. Then the ray becomes more or less parallel, but its section becomes oval, so all the spots of the diodes on their mirrors are elongated in the direction orthogonal to the picture plane. It means, that one can place the rays even more tightly, than it could be if the spots were round. Particularly the collective beam of the mentioned above 4 pcs NUBM08 diodes has 6x3 mm section at the beginning, so the intensity is equal to 83 W/sq. cm. without any kind of focusing.

 

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