Photonics West 2025

[Martin]

What about that one?

13390-30
Volume Bragg gratings for locking and beam combining of blue diode lasers
Author(s): David Guacaneme, Oussama Mhibik, Ivan Divliansky, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States)
29 January 2025 • 2:10 PM - 2:30 PM PST | Moscone West, Room 2006 (Level 2)

Abstract: A volume Bragg grating (VBG) is used to spectrally lock a blue laser diode externally to achieve linewidth on the order of tens of picometers. In conventional laser diodes the emission linewidth tends to be on the order of several nanometers due to the short photon cavity lifetime, which is a limitation for many applications. In this work, we manufacture a VBG that is holographically recorded to form a notch reflecting filter and is used to externally lock a blue laser diode reducing its emission to tens of picometers. This external cavity locking method provides narrow laser linewidths from conventional laser blue diodes while keeping a compact system design. We extended the approach to coherently combine several blue diodes in order to increase the total output power.

I assume that approach might be doable with red laser diodes as well...

[Joe Farina]

One very practical difference is luminous efficacy of those wavelengths, i.e., how bright they look like. If we set luminous efficacy of 555 nm as 100%, then luminous efficacies of 610, 640 and 660 nm are 50%, 17% and 6%. In short, a bright red object would appear much dimmer with 660 nm than with 610 nm.
Note that those per cent numbers do not exactly describe perception, they just tell CIE Y value. A more appropriate description would be using L* value of CIELAB color space. For 555, 610, 640 and 660 nm, the L* values are 100, 76, 49 and 30. So, quite a significant drop from 640 to 660 (or even from 610).

Thank you, that's very interesting and helpful.

Having done some tests with colors and color holograms, I've been surprised (many times) about color behavior. For instance, when using 488, 532, and 633nm, I would have expected the blue color rendition to be almost non-existent. Attached is the 1976 CIE diagram with those colors forming the triangle.

cie1976.jpg (34.84 KiB) Viewed 2245 times

While those wavelengths do result in poor rendition for many blue colors (pigments), I was surprised that the pigment Cobalt Blue does surprisingly well. It reproduces nicely as a sky-blue color. This was unexpected, based on the CIE diagram.

What about that one?

13390-30
Volume Bragg gratings for locking and beam combining of blue diode lasers

Thanks Martin. I hope that research has some practical results, from our standpoint

[Din]

I assume that approach might be doable with red laser diodes as well...

I think these Bragg gratings would have to be very narrow band. So, whether it could be possible with red diodes would depend on whether bandwidth is proportional to wavelength.
I'm a little intrigued by:

We extended the approach to coherently combine several blue diodes in order to increase the total output power.

Wouldn't all the blue diodes have to have the same spectral profile within the bandwidth?

[Din]

While those wavelengths do result in poor rendition for many blue colors (pigments), I was surprised that the pigment Cobalt Blue does surprisingly well. It reproduces nicely as a sky-blue color. This was unexpected, based on the CIE diagram.

Assuming you're recording DCG, it could depend on how you process. Display DCG always shifted to a lower wavelength due to differential swelling, so the 488 might have shifted down to a lower band; sky blue seems to be about 480.

[Joe Farina]

Assuming you're recording DCG, it could depend on how you process. Display DCG always shifted to a lower wavelength due to differential swelling, so the 488 might have shifted down to a lower band; sky blue seems to be about 480.

Yes. That's true, I've seen a lot of variation in the final hologram due to shrinkage. But just illuminating the objects with laser light at 488/532/633 results in a very nice blue when Cobalt Blue is used.

[lobaz]

That CIE 1976 UCS diagram is quite misleading, Joe, at least if you are watching it using a common sRGB display. Displaying chromaticity diagrams is always tricky as most of its area cannot be displayed using a common display. Thus, many colors are just false. Anyway, the diagram you have posted is off quite a lot, as it indicates 488 nm as mostly green. This is a better image:

CIE1976UCS.png (44.52 KiB) Viewed 2230 times

I have simulated what happens with 488, 532 and 633 nm. Suppose you turn the red laser on at power R (0 = off, 1 = full on), the green laser at power G and the blue laser at power 1 - R - G. Additionally, suppose laser maximum powers are balanced so that turning all of them at approximately max results in white. Achievable colors then look like this:

laser488+532+633diagram.png (29.76 KiB) Viewed 2230 times

The colors inside the clipped triangle around white should be displayed correctly using sRGB display. Colors outside are false and just for reference.
Anyway, as you can see, cyan colors such as sky blue make no problem. Royal blue as well as purples may be poor. However... When assessing color, one has to differentiate between "unrelated color", i.e., a color partch without any surroundings, and "related color", i.e., a color in image. Color perception changes a lot when looking at images -- although colorimetrically there may be no royal blue, you may perceive the image as it was there.

[Joe Farina]

Thanks, Petr, the analysis is much appreciated. I noticed variations in the reproduction of the CIE diagram before, but didn't realize how much they varied until you pointed out how inaccurate that one was. The simulation you posted (the second diagram) looks like what I've experienced so far with 488/532/633.

Regarding practical lasers for color holography, it's too bad that blue and especially red are problematic. As more people nowadays are using the Bayfol material, it becomes more important. Silver is of course very fast, but many people seem unwilling to do wet processing. I'm working on color DCG, but progress has been slow.

[Din]

One peculiarity about display holography is that it exists in two different modes. When you're recording a hologram, you're in the radiometric mode, while if you're looking at a holographic image, you're in the photometric mode (https://www.thorlabs.de/catalogPages/506.pdf ). This means that when recording a hologram, the recording process is evaluated in terms of wavelength and power because the recording is based on the material sensitivity in terms of an energy/wavelength relationship and the appropriate actinic reaction. However, when viewing a hologram, appearances are important as they relate to vision, as Petr and others have noted. So, perhaps the important factors are the photopic curve and Land white correction among others and the visual relationship between neighbouring colours, again, as Petr pointed out. Perhaps, the image ought to be measured in lumens, as opposed to power, bringing in the photopic curve. There are also psychological factors. As some references have pointed out, if you see a photograph of a grassy lawn, the grass always seems lush, but you have no idea what the original lawn looked like - you expect lawns to have lushy green grass. In a colour hologram of human dolls, you expect the face to have flesh-like tones.

By the by, I had the VP of Hewlitt Packard's (HP) printer division at our lab. So, of course, him being an expert on colour, I had to grill him about colour. One topic I was always interested in is, how many subjects were used to create the CIE (bearing in mind, the CIE was designed for printers). He said, "Twenty white guys in Paris". Really! Just 20 white guys in Paris? This made me wonder what if they'd used women, or people from a different culture. But, I digress

[Joe Farina]

However... When assessing color, one has to differentiate between "unrelated color", i.e., a color partch without any surroundings, and "related color", i.e., a color in image. Color perception changes a lot when looking at images -- although colorimetrically there may be no royal blue, you may perceive the image as it was there.

However, when viewing a hologram, appearances are important as they relate to vision, as Petr and others have noted. So, perhaps the important factors are the photopic curve and Land white correction among others and the visual relationship between neighbouring colours, again, as Petr pointed out. Perhaps, the image ought to be measured in lumens, as opposed to power, bringing in the photopic curve. There are also psychological factors. As some references have pointed out, if you see a photograph of a grassy lawn, the grass always seems lush, but you have no idea what the original lawn looked like - you expect lawns to have lushy green grass. In a colour hologram of human dolls, you expect the face to have flesh-like tones.

Thanks. I think I've noticed this. Maybe it's an example of wishful thinking made "real." Or maybe previous experience overriding objective appearances.

[Martin]

Thanks Martin. I hope that research has some practical results, from our standpoint

Ondax, now Coherent, have been selling that kind of laser systems for more than ten years...