The conversion is not that easy and depends on pump diodes and laser crystals which are heated or cooled. Efficiency and performance of a DPSS module highly depends on temperature, which also affects the beam quality.
Differences between DPSS laser sources and laser diodes
A disadvantage of DPSS laser sources are their sensitivity and the modulation behavior (due to the assembly of the crystal). There are high-end DPSS laser sources, which allow a very good beam profile and modulation thanks to several temperature control loops and perfect optics. This however is not the case with many cheaper DPSS laser sources; the internal temperature of the DPSS module influences the beam profile and modulation behavior resulting in bad beam profiles and power drops in modulation.
The DPSS laser sources do have some advantages which are low price, compact and can have high output power. Another advantage up until now is that there were no alternatives to Green DPSS laser sources (apart from Coherent OPSL laser sources, which are not suitable for smaller projectors and lower cost options).
A couple of years ago there had been a similar revolution with the change from blue DPSS laser sources to blue Diodes. The former blue DPSS laser sources had another disadvantage: their efficiency was only half as good as the efficiency of green DPSS laser sources.
Advantages of Blue laser diodes:
- Laser Diodes are much smaller and more compact than a DPSS module - a diameter of 5 to 9mm for a blue laser diode, which has an output power of approximately 1W compared to a blue DPSS module with an output power of 0.5W at a wavelength of 473nm, measures 155 x 77 x 60mm! Last but not least, the corresponding PSU (also known as a driver) measures 238 x 146 x 94mm!
- Laser diodes don’t require such a complex stabilisation of temperature compared to the DPSS crystals, DPSS laser sources need to be cooled or slightly heated (if the projector is too cold). Inside DPSS laser sources up to three temperature control circuits apply: the first one for the pump diode (the variation of the normal operating temperature negatively influences the wavelength), the second one stabilises the laser crystal (mainly Nd:YAG) and the third one stabilizes the “doubler” (KTP or LBO, depending on the construction). If one of these temperature loops isn’t correctly adjusted, it results in deviations, flickering and power drops.
- Laser Diodes have no movable parts, such as the output mirrors in a DPSS module. The structure of the laser diode (construction as a semiconductor) also acts as the laser cavity. This is why they can’t be disassembled as DPSS laser sources. Vibrations and hard shocks often lead to drops in output power and in worst cases to a complete blackout of the laser source which results re-adjustments being required.
- Laser diodes are easier to modulate than DPSS laser sources, as the laser diode current is lower than on a DPSS source. Due to their build-up, Laser diodes are much more efficient than DPSS laser sources and therefore produce less heat waste and simultaneously less power consumption.
- There is one disadvantage of Laser diodes: the low power output. Currently, laser diodes are limited to an output power of 120mW to 1.5W, depending on the wavelength. This leads us to ask: “How to get more power?”
- By simply coupling laser diodes it is possible to achieve more power, which on the other hand is limited by the beam profile of the single diodes as you can’t stack as many diodes as you wish (diameter and Divergence; FAQ: Knife Edge)
Similar to the arrival of the Blue laser diode, the development of the green laser diode changed the laser market. Many laser projectors have been re-built and even re-designed at Laserworld, to maximize and use their full potential.
What advantages does the pure diode projector have in comparison to the “Diode-DPSS” projector?
Due to the extremely compact laser diodes, it is possible to have high power in extremely small housings, the best example: our PM-3800RGB Pure Diode. Even better – the new RTI ATTO RGB 2.5.
- Divergence is another important aspect of the show laser industry, the lower the divergence the sharper the laser beam will be. Low Divergence is essential for high quality graphic projections, through sharp and accurate projection lines.
- Power consumption and heat generation can also be drastically reduced, thanks to the diode technology – on Pure Micro projectors the power consumption has been divided by half.
- Modulation-behavior: This aspect is the most important one for laser show programmers and designers. Thanks to linear modulation, the starting colours can easily be adjusted; flickering and power fluctuations belong in the past.
- The semiconductor technology also increases the life span of a laser source, as there are no movable parts integrated. The problem of the cold-sensitivity does not apply, because there are no laser crystals used in the construction (Nd:YAG/KTP): Laser Diodes can be used also with lower temperatures – this is not the case with the temperature dependent DPSS laser sources:
- Wavelength of the pump diodes drifts with temperature -> if the DPSS module is too cold, the pump diode only outputs 804-802nm, not the full 808nm.
- The Nd:YAG crystal (Neodym Yattrium Aluminium Granated) reaches its full efficiency at 808nm pump wavelength. If this changes, the efficiency collapses which results in much less power at 1064nm.
- The temperature of the frequency doubled crystal needs to be stabilised, because otherwise its efficiency collapses.
The past developments within the show laser industry have shown that semiconductor technology has become an important integral part.
NLO: non-linear optics
Knife Edge: technology to couple many laser diodes to one bundled beam
Divergence: “Broadening” of the laser beam
DPSS: Diode Pumped Solid State
Nd:YAG: Neodym Yattrium Aluminium granated: turns 808nm -> 1064nm
KTP: Frequency Doubling Crystal. Turns 1064nm -> 532nm (Green)
OPSL: Optically Pumped Semiconductor Laser