Mode Stability of Diode Lasers

A growing collection of notes, focusing on diode mode stability measurements.

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1) Summary

2) Capturing mode hops

3)  Longitudinal mode analysis   

4)  Digital control

5) Results for the Rohm RLD65PZB5 80mW/658nm laser diode

6) Results for the Mitsubishi ML120G21 80mW/658nm laser diode

7) Results for the "long open can" Mitsubushi ML101U29 150mW/660nm diode

8) Results for the Sony SLD1239JL-54 100mW/658nm laser diode

9) Results for the Sony SLD1236VL 80mW/658nm laser diode

10) Results for the Mitsubishi ML101J27 130mW/660nm laser diode

11) Results for the Opnext HL6385DG 150mW/642nm laser diode

12) Results for the Sharp GH04P21A2GE/PHR-803T 100mW/406nm "blu-ray"laser diode

13) Results for the Mitsubishi ML101F27 150mW/660nm laser diode

14) Results for the Nichia NDB7412 1W/445nm laser diode

15) Results for the Mitsubishi ML520G71 300mW/638nm,       HL6388MG  250mW 637nm Opnext laser diodes

16) Results for the Opnext HL63133DG/170mW/638nm laser diode

17) Results for the Osram PL450 80mW/450nm laser diode

18) Results for the Opnext HL45023TG 80mW/445nm laser diode

19) Results for the Opnext HL63603TG 120mW/638nm laser diode

20) Results for the Mitsubishi ML520G54/110mW 638nm laser diode

21) Results for the Osram PL520 50mW/520nm laser diode

22) Results for the Mitsubishi LPC-836 300mW/655nm laser diode

Local links:

Home-build instrumentation:


Off-site links:

 


1) Summary

Here is a summary of my measurements. Note that most measurements (except for the Rohm diode) were done with one or two diodes only, and there may well be significant differences between diodes of the same type;  so this is meant just as a rough indication only.

Diode Wavelength P(multimode) P(single mode) P(ECDL) Remarks Summary
RLD65PZB5 658nm 100mW @ 170mA 30-40mW @ 80mA bad prob needs recheck not very interesting
ML120G21 658nm 80mW @  140mA 30mW @ 80mA ? weak extra modes  at higher power reasonable
ML101U29 660nm 250mW @ 300mA -- >100mW @ 220mA windowless  ECDL only
SLD1239JL-54 658nm 120mW @ 170mA 20-30mW @ 70mA bad windowless not interesting
SLD1236VL 658nm 100mW @150mA 20 mW @ 60mA ?   not interesting
ML101J27 660nm 130mW @200mA 40-90mW varies strongl >100mW @ 160-200mA >80mW SM at narrow spots good overall value
GH04P21A2GE 406nm 100mW @110mA -- 15mW@50mA very bad free running   ECDL only
HL6385DG 642nm 140mW@290mA >75mW@175mA@ 15C >100mW@230mA excellend for ECDL very good
ML101F27 660nm 150mW@225mA -- bad   not interesting
NDB7412 445nm 1W @ 1A 64mW@233mA@14.9C 80-200mW, unstable not TEM00 bad beam quality, unstable

ML520G71  HL6388MG

638nm     637nm

300mW    250mW

-- bad not TEM00 bad beam quality, useless
HL63133DG 638nm 170mW approx 50mW at 16C >100mW

moderately ok if free running

ECDL best diode
PL450B 450nm 80mW ---- 40mW@110mA very bad free running,3.8mm  ECDL only
HL4503TG 445nm 80mW       TBA
HL63603TG 638nm 120mW <10mW 70mW@125mA mediocre free running,3.8mm ECDL excellent
ML520G54 638nm 110mW <10mW 60mW mediocre free running,3.8mm ECDL excellent
PL520 520nm 50mW --- 40mW@150mA very bad free running ECDL good
LPC-836 655nm 300mW -- --- windowless useless

In a nutshell:

As of Jan 2014, for SLM operation the best price-performance ratio for red diodes in ECDL mode are the HL63603TG and ML520G54. The most powerful is the HL63133DG. Reasonably good free-running SLM diodes are HL6385DG and ML101J27 . Green, blue and violet diodes generally run strongly longitudinal multimode when freely running and so are not suitable for holography, however some of them run well in ECDL mode.

Totally unsuitable are transverse multimode diodes such as ML520G71 , HL6388MG, NDB7412, and also high power transverse single mode like LPC-836. If a diode runs multiple tansverse mode to begin with, there is little chance to have it running in single longitudinal mode operation (though with lots of effort some marginal SLM operation at low powers is sometimes possible, eg for NDB7412.

 


2) Capturing Mode Hops via noise in laser output

One way to get a handle on laser diode stability and mode jumps is to look for noise in the light output; this has been described here, which I recommend for more background information. My hardware setup includes a large area photodiode behind an OD3 neutral density filter, which feeds a simple opamp acting as transimpedance amplifier, then an AC amplifier, and then a RMS detector based on the LTC1967. This is then optionally followed by a sample-and-hold circuit and a digital data acquisition unit (see below). I may post the circuit(s) at a later time.  

On a scope, the output then looks typically as follows:


Left pic: Current scanning. Upper trace shows raw input signal. Lower trace shows signal after DC rectifying and peak detector. Note the small periodic peaks in the input: they arise from the jumps in light intensity when the laser diode current is periodically stepped up by 1mA. These  peaks do not make it through the signal conditioning circuit, though.
Right pic:  Temperature scanning. Upper trace shows raw input signal. Middle trace shows signal after after RMS-to-DC converter LTC1967. Bottom trace shows signal after a sample-and-hold circuit which is necessary in order to capture the quickly varying signal with a low speed digital data acquisition unit (see below); the periodic ramps are the integrated signal, and just before when it is reset (ie at the peaks of the ramps), the signal is read out to an ADC for digital processing. This integrated signal is a good measure for the noise and also captures brief signal bursts that would not be visible by the ADC because it is read out only once per second.

We observe that on the one hand (right) there can be sudden bursts of high intensity noise, indicating a transition through chaotic regimes, and on the other hand (left) there can be simple mode jumps where the output power suddenly jumps by a discrete amount but otherwise stays clean. My signal conditioning circuitry has been specifically designed to cope with this behavior. Further below I have correlated these measurements with spectrum analysis.


3) Longitudinal Mode Analysis via Optical Spectrum Analyzer

In fact, it turned out that just looking for noise in the light output is not sufficient to determine whether the laser diode is suitable for holography, at a given operating point. This is because noise just signals changes in the mode power spectrum, but even if the mode spectrum is stable and there is no noise, it is still unclear whether just one or several modes are present. For holography, one needs the diode laser running a single longitudinal mode only. The mode spacing for a diode laser is so large (in the order of 100Ghz or more), that the presence of any additional mode would bring the coherence length down to a mm or so, and so ruin any hologram by giving it a "sliced bread" appearence.

The tool of choice for determining the mode spectrum of a laser diode is a grating based optical spectrum analyzer. Given the mode spacing of 100Ghz+ of a laser diode, it is better suited for displaying the spectrum than a scanning Fabry-Perot interferometer, because the latter has a useable free spectral range of typically just a few Ghz.  In other words, the spectrum of a diode laser would "wrap around" many times, which would make it difficult to unambiguously identify the modes.

Click on the pic below for a brief movie that shows mode instabilities that occur when the laser diode current is increased by a few milliamperes.
This has been recorded off the scope screen, displaying the CCD output of my (mostly home-mode) optical spectrum analyzer.
Note that a single mode zone is crossed after a few moments:

=

The resolution of the still picture is approx 0.12nm/div, and of the movie 0.3nm/div.

Moreover, I filtered the output of the spectrum analyzer circuit through a MAX9141 high speed comparator that triggers at an adjustable level. In this way one obtains an "effective line width" which measures the total range spanned by all the longitudinal modes. After an RC filter this then gives a DC voltage which is proportional to the total number of longitudinal modes that are present. Here a pic of how the comparator (output shown on the lower trace) acts such as to provide a voltage that is proportional to line width (rather than output power):


4) Digital Control

In order to facilitate a systematic analysis, I was setting up a machinery for automatically scanning over laser diode temperature und current, using a digital USB interface based on the IOWarrior chip (a recycled left-over prototype from my Coherent315 DPSS controller project, the circuit being very similar to this one). Among other things, this consists of DAC's to provide input voltages to the TEC controller and the LD driver, plus a couple of ADCs for measuring the output power, light noise and line width after processing the signal by the conditioning and sample-and-hold circuits mentioned above.

On the software side, I made a LabVIEW program for scanning the temperature/diode current plane (available upon request). The measured AC noise and linewidth data of the laser output is written on a file for graphical post-processing.  Here a look of the GUI:

Top diagram: Effective line width obtained from the spectrum analyzer.
Second diagram: RMS noise of light output.
Third diagram: red line = actual thermistor voltage, white line = control voltage of TEC controller.
Note the steps by one mV every time a full scan of the diode current is completed.
Bottom diagram: TEC current monitor. There are spikes from the PID feedback loop whenever the setpoint voltage is stepped up by one mV.

 


5) Results for the Rohm RLD65PZB5 80mW/658nm Laser Diode

Its data sheet is here.

Several thousands of those have been up on ebay recently for less than $10 each, so obviously they will be popular among laser hobbiests and deserve special attention from the holographer's viewpoint. They are rated at 80-100mW CW but pulse peak power is 240mW, so optically they should be able to do more than 100mW CW (perhaps with reduced lifetime).   

Indeed so - I found for the first diode I tried that at 170mA and 15 degrees C, the output was already 108mW!  This even with collimator optics, without them probably a few percent more. I didn't try to crank up the current more, though. The wavelength turned out to be at approx 654nm at 15C, which is not too bad either. The next two samples ran at approx 655nm (with similar output power) while others reported a wavelength of 661nm, so there seems to be some variance. At any rate, the wavelength shifts by approx 0.25nm per degree centigrade, and this needs to be taken into account.

The question is whether there are useable zones at reasonable power which are single-mode and stable, ie. mode-hop free. A systematic current/temperature "landscape" analysis of output noise was done, using the machinery described above, to see where stable and unstable zones of a given diode are.

Spectrum analysis

Below on the left there is a landscape plot of the (effective) line width, which was obtained from the CCD spectrum analyzer via the digital machinery as explained above.  It took almost a full day to complete...  blue signals single mode, and we see that it does not occur much beyond 90-100mA. This corresponds in the best case to slightly more than 50mW, but typically the Rohm diode can do approx 30mW in single mode. At higher powers the diode runs strongly in multimode.

On the right there is, for comparison, a noise plot obtained in the same run. The grey regions indicate weaker and the black regions stronger noise (on a logarithmic scale -- the light grey regions correspond to a few mV of noise and the black regions to several hundreds mV). While there are obvious correlations between these plots, we see that low noise is not sufficient to guarantee a clean mode spectrum; as already noted, noise signals changes in the mode spectrum but does not really tell much about the mode structure as long it remains stable. From this it is evident that the noise scans are not too useful for determining a laser diode's suitability for holography!

Things should look different for each diode, and even for the same diode when removing and re-mounting it (because what counts is not the heatsink temperature, but the actual diode die temperature, which depends on the precise heat transfer characteristics between diode case and heat sink. Also adjusting the collimator lens can have an effect, as I found out).  Note also that noise occurs where chaotic mode competition occurs; simple mode jumps can and will occur in any place when the temperature is changed, and preventing them over an extended period of time, requires to hold the temperature constant to a fraction of a degree.

Below there is a representative summary of an extensive analysis, where I have superimposed the line width and noise scans and plotted the actual spectrum at a number of selected points (click for a larger 2MB .pdf copy):

The little boxes display on the upper trace the spectrum at ca 0.12nm (85Ghz) per div, and the noise output on the lower trace.
The following observations can be made:

Thus, not surprisingly, the rule seems to be that whenever there is the slightest noise in the laser power, there is multimode behavior to some degree.
On the other hand, absence of noise does not guarantee the spectrum to be single mode; it just tells that it is stable.


6) Results for the Mitsubishi ML120G21 80mW/658nm laser diode

 

Its data sheet is here.  I obtained with no problem 82mW at 140mA at around 14 degrees C, even after collimator. I ran similar measurements as above, with the following summarizing results for linewidth (colored) and noise (superimposed grey/black) as function of temperature and laser current:

What is interesting is a relatively stable zone with small linewidth at low temperature and high currents; the power there is close to 80mW and thus is considerably higher than for the Rohm diode.  Concretely I have re-checked the region near 13.5 degrees and 130mA with the scanning interferometer and the result is as follows:

Upper trace: signal from CCD spectrum analyzer, scale is approx 0.12nm/div = 85Ghz/div
Middle trace: signal from scanning Fabry-Perot interferometer, scale is approx 1/100 of the above, say 0.001nm/div = 300Mhz/div.
Bottom trace: ten-fold zoom of middle trace, which is approx 30Mhz/Div. Line width is thus at most 10Mhz, which means a coherence length of at least 30m.

This zone is not strictly single mode, as we do seem to see a few extra modes, but they are very weak so that they probably wouldn't harm a lot. This operating point at close to 80mW definitely deserves a test shot of a real hologram!

Thorlabs sells it as single mode diode, and as we see that its spectral data are definitely better than the other DVD diodes I tested; there are indeed large stable single mode zones but this does not exclude multi-mode zones to exist in-between.

 


7) Results for the "long die open can" Mitsubishi ML101U29 150mW/660nm diode

  This kind of diodes, for which the die is in the open, can achieve very high power, in excess of 250mW CW.  See e.g. here for some information. I obtained a few of such diodes, and first confirmed these measurements: at 16C the laser threshold was at about 72mA, at 200mA the output was approx 100mW, and rising to 225mW at 380mA. This was using a simple Aixiz uncoated plastic collimator, with their coated glass collimator the power is approx 5% higher.

I don't really know the make of these diodes, but they should at least be similar to the long open can Mitsubishi ML101U29, which ``officially'' boasts 150mW CW output power; people report to have achieved more than twice as much, but that's a gamble and most likely dramatically reduces the lifetime of the diode.

At any rate, as for the mode spectrum I didn't expect any surprises, and in fact there weren't any. In short, these diodes are totally unsuitable for holography, as their spectrum is just terrible. Here is a linewidth scan over current and temperature:

There is almost nowhere a single mode zone, not even at low currents. Below some representative shots, made via a LabVIEW interface to my CCD spectrum analyzer:

(I=180mA,T=16C)

(I=250mA,T=16C)

(the labeling is not correct, it is rather 0.12nm per 20uS).

On the other hand, I found in an ECDL setup this diode is well capable of obtaining 100mW single mode - see here for details.

 


8) Results for the Sony SLD1239JL-54 100mW/658nm laser diode

This is a diode currently easily available on ebay. It is also of open can type, ie, there is no window and the bare die is exposed - this needs a very careful treament.  In multimode it can run to quite high powers, in excess of 200mW; I obtained approx 120mW at 170mA. Let's see what the mode analysis tells:

Colors indicate line width and the gray overlay AC noise in the laser output.
(Note that the colors should not be compared across different plots due different normalizations; for the two Sony diodes, green and cyan indicate single mode zones.)

So without any further discussion it is clear that the diode won't work well beyond approx 70mA, which yields approx 25-30mW. Even then there are mode hopping zones and one needs to carefully find a large enough stable zone.

I also checked an ECDL version and it didnt work well at all.


9) Results for the Sony SLD1236VL 80mW/658nm laser diode

This diode is similar to its more powerful sister above; in multimode I got 102mW out at 150mA after collimator. Here the result of spectrum analysis:

Like most DVD diodes I checked, it runs single mode only up to 60-70mA which yields 20-30mW. It seems more noisy with lots of mode jumps but that may also simply be an artefact due to a different noise voltage calibration, the pictures are not meant to be compared to each other.


10) Results for the Mitsubishi ML101J27 130mW/660nm laser diode

With the first diode, I achieved 38mW at 100mA, 81mW at 145mA, and 123mW at 190mA; with another I got 133mW at 190mA. Thorlabs lists this diode as single mode, though there is no mention of this in the data sheet. As for the landscape scan, I improved the signal processing circuits such as to directly count the number of modes and this (nearly) independent from the output power. Thus the plot looks slightly different to the ones before, but is more meaningful for cross-comparisons; cyan corresponds to single mode, defined by that there is no other mode present with more than 10% power; blue denotes two modes, and so on:

A sweet spot looks like this under the CCD spectrum analyzer (the following plots are for a different diode with slightly different pattern):

(I=110mA, P=48mW,T=16.6C)

(the labeling is not correct, it is rather 0.12nm per 20uS).

A check with the scanning interferometer shows undeniable single mode with a line width of a few Mhz:

 

I noticed a large variation between different diodes, even out of the same batch; for three diodes I found single mode behavior at powers 50...65mW, one could do single mode only up to approx 40mW, and the last one was a surprise: approx 90mW at 140mA (however the single mode zone is quite narrow). Here the plots of the last two diodes I mentioned:

All-in-all, this diode has be best price/performance ratio of all I tested so far (unless one happens to get a bad exemplar such my #4). It performs also well in an ECDL configuration, but the problem of narrow zones is even more pronounced here.

 


11) Results for the Opnext/Hitachi HL6385DG 150mW/642nm laser diode

 

I bought a few of these due to a special offer from Photonic Products. As per data sheet, these diodes are classified "single longitudinal mode" and this, of course, strikes an holographer. Apart from the high power, also the wavelength is interesting, it is of a much warmer red than the usual 658nm diodes have, and this is of better use for color holography, the film sensitivity is higher, and also waveplates for 633nm HeNe are likely to work better as well. Here the landscape scan, where cyan corresponds to single mode, blue denotes two modes, etc:

The good feature is the single mode zone at high power, up to more than 100mW, but it occurs at a in quite narrow band, and the challenge is to keep the diode running in such a band for long times without drifting out.  Another challenge is to avoid moisture condensation at the required low temperatures. It may require sealing of the laser head.

For the second and third diodes I tested, the plot look quite different:

    

 I also checked single mode zones in more detail with the scanning interferometer. Indeed there can be a fine structure that the CCD spectrometer cannot resolve. Above I indicated three locations and the corresponding SFPI spectra; in each display the upper trace shows an FSR of ca 2Ghz, and the lower trace, a tenfold zoom. That is, the scale on the lower trace is approx 20Mhz/Div. We see that at spot C the spectrum is noisy and this indicates an instability. Still the line width of ca 40Mhz should correspond to a coherence length of like 10m. I found that the spectrum is generically much noisier at high powers, and to what extent that matters in a hologram remains to be seen.

I now also tried that diode in an ECDL setup and the preliminary results are here.

 


12) Results for the Sharp GH04P21A2GE/PHR-803T 100mW/406nm "blu-ray" laser diode

This is an interesting little beasty close to the UV with a lot of power. My sample had threshold current at 35mA and achieved 93mW at 100mA and 20C, which is well consistent with these findings. Actually one should be able to tickle more power out but I didnt want to risk the diode before taking measurements. The power was measured behind a C330TM-A Geltech collimation lens which is AR coated from 400-600nm. With an Aixiz glass lens coated for red lasers the power was 70mW only, at 100mA (actually it is hard to measure power accurately with photodiode detectors at the edge of their calibration domain; I measured ten percent less power with my Newport 835/818-SL as compared to my Scientech and Laser Precision thermal power meters).

Unfortunately the spectrum turned out among the worst I ever have seen, it is strongly multimode almost everywhere in the current/temperature plane:

This is entirely consistent with the graph that is shown at the end of the data sheet. Here a few representative samples, all taken around 16C temperature (the horizontal axis was not properly scaled):

  I=38mA ... multimode even just above threshold!

I=70mA

I=100mA

We see that with higher currents the higher total power comes from having more and more modes, and not from a single mode becoming all the time stronger. Let's crudely observe that each mode never gets beyond an order of 10mW.  That seems to be the same order of magnitude that single mode ECDL lasers achieve; for my findings, see here.

 


13) Results for the "open can" Mitsubishi ML101F27 150mW/660nm diode

This diode is supposedly an upgraded version of the ML101J27, and I just got a couple of those. Indeed the power is higher, I got 150mW at 225mA with a Lens-27 with the first sample I tested. It is an open can diode like the ML101U29 and has similar specs. Indeed, my tests showed that it runs mostly multimode over the operating region:

However, again similar to the ML101U29 it runs moderately moderately fine in an ECDL configuration.


14) Results for the Nichia NDB7412 1W/445nm laser diode

There has been incredible frenzy over the blue laser diodes that can be harvested for like $30-50 from a Casio A130/140 projector; see the turmoil at the various light show and ballon popper forums. There one can find further details about all sorts of things like beam profiles etc; in particular here. It seems that these diodes have the same data as the Nichia NDB7412 1W 445nm diode; despite of that the kind of these diodes is not yet entirely clear, I denote these diodes tentatively by NDB7412.  

One drawback is the quite ugly transverse mode profile, and it will take quite some effort for beam shaping to e.g., feed the light through a spatial filter efficiently. First beam cleaning tests were not promising; I intend to add a page on collimation, beam shaping and handling the transverse mode structure when I have gained more results and insights.

The holographer is of course mainly interested in the mode stucture of these little beasties. Within the hour of receiving my first diode, I set it up and checked the mode structure with my home-built CCD spectrum analyzer (due to lack of a 1 Amp capable driver I had first to modify my SDL800 such as to allow it to run blue diodes with higher forward voltage). Et voila - without any systematic search, I immediately found that the diode runs single longitudinal mode at 55mW (with Lens-27 at 221mA and 15.4C) !

This was truly surprising, not the least because the diodes run transverse multimode. Some research of literature revealed that this mode structure typically arises for broad emitter diodes due to "filamentation" of the mode pattern, forming so-called extended "supermodes" which lase at the same frequency. These display a characteristic far-field pattern which arises from interference of coherent superposition.  It seems that the situation we observed is exactly the same as the one discussed in this article on "Supermodes in Broad Ridge (Al,In)GaN Laser Diodes".

Here is a bunch characteristic samples we found:
Nichia-spec
From top to bottom the current was 221mA, 225mA, 300mA, and 400mA at 16C. We see that at low powers, just one or a few supermodes are present, but at higher powers there is then chaotic competition of a broad multitude of supermodes. Note the beautiful single mode at 221mA!  

Here is a little movie that shows the spectrum when the current is ramped from threshold to approx 260mA at 16C; note the beautiful single mode zone at 224mA:

Included is also sound that shows the noise in the light output. The popping sounds at the beginning indicate mode jumps. One can clearly hear how the single mode regions are correlated with low noise, which gives hope that a simple noise detector might be sufficient to determine the single mode regions.

Here the result of the first landscape scans, it was indeed a very pleasant surprise; as always, cyan denotes single mode behavior and we also have superimposed elevated noise in grayscale:

The first diode is somewhat better behaved than the second, and both are much better than the third. I checked a sweet spot of the first diode at 233mA and 14.9C to yield 64mW. This region is quite narrow, though, so it might be better to prefer a safer region. We see that the diodes differ quite strongly and I need to make more tests in order to see the typical behavior.

Here power plots of the diodes I tested so far; they can be pushed to beyond 1W but I didn't turn them higher because single mode cannot be expected at high powers anyway:

A first test in an ECDL configuration was disappointing but subsequent ones with a different grating were increasingly better, see here. It seems the over 200mW SLM is possible in an ECDL configuration, but only to approx 60-80mW is reasonably stable.


15) Results for the Mitsubishi ML520G71 300mW/638nm and HL6388MG 250mW 637nm Opnext laser diodes

These are high-power broad-stripe diodes which also run transverse multimode with a very bad beam profile. I couldn't find any longitudinal single mode spot, no matter what current or temperature were. One of the worst diodes for holography, and ECDL mode ist not much better. Not worthwhile to present any further measurement data.


16) Results for the Opnext HL63133DG  170mW/638nm laser diode

This is a narrow-stripe single transverse mode diode, which is similar to but somewhat better then the proven HL6385 (and definitely more expensive). It may be the best diode for holography operation, when disregarding the price. Here a plot of the free-running diode:

Blue zones depict single longitudinal mode operation (or close to it), and the power is a few tens of mW. Not bad! 

Here a nice clear spectrum close to HeNe wavelength, at around 50mW and 16C:

However, in ECDL mode the diode totally rocks!


17) Results for the Osram PL450 80mW/450nm laser diode (3.8mm case)

This diode was sold on ebay as the Opnext HL45023TG. It has quite similar data, so most wouldn't complain, but for purposes of single longitudinal mode operation there may be a major difference. We will see!

While the case is slightly different, it is still of the 3.8mm type and that created some headache - how to mount it precise and stable enough while having good thermal coupling to the temperature controled mount. Something like 0.5W is going to be dissipated, and the surface area for a press mount seems minimal. When I did the test, pre-fabricated 3.8mm housings didnt exist, so I decided to solder the diode. This requires a low-temp solder, fortunately I could get some chunks of Woods Metal from our cryogenic workshop, the melting point of that particular alloy I got was indicated to be at 71C. So I fixed a little temperature controlled case on top of my hot-plate setup, which has a precise TEC control.After some attempts with dead dummy diodes, I finally managed to solder the tiny 3.8mm diodes to a small brass bar that fits onto my standard diode mounts, while keeping the temperature at 75C max:


The diode survived it, performing flawlessly up to specs; I didn't chase it beyond 60mW, which was achieved at around 120mA. The beam quality was pretty good, much better than for the 1W Nichia.

However disappointing was its behavior with respect to SLM operation; free-running it was among the worst ever seen, not the slightest chance of a SLM spot, quite similar to the 405nm blue ray diode. Here a characteristic plot of the spectrum at around 60mA:

After tinkering, the diode performs fine in ECDL mode up to at like 40mW, for results see here.

 


18) Results for the Opnext HL45023TG 80mW/445nm laser diode (3.8mm case)

Diode died, stay tuned.


19) Results for the Opnext HL63603TG 120mW/638nm laser diode (3.8mm case)

This diode does not run too well when without feedback, no systematic tests done.
Performs excellently for ECDL; see here. Very good price performance ratio.  


20) Results for the Mitsubishi ML520G54/110mW 638nm laser diode

This diode does not run too well when without feedback,no systematic tests done.
Performs excellently for ECDL; see here. Very good price performance ratio. 


21) Results for the Osram PL520 50mW/520nm laser diode (3.8mm case)

Absolutely unsuitable when running free:

But it is pretty good in ECDL mode; see here


22) Results for the Mitsubishi LPC-836 300mW/655nm laser diode

Totally unsuited for single longitudinal mode operation in either free-running or in ECDL setup. Not worthwhile to present any further measurement data.


 

More diode tests will follow over time!  

 


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Vers 4/1-2014