06 Apr 2009
Exclusive to optics.org: Innovative intra-cavity design has produced a continuous-wave ruby laser, with other novel sources set to follow.
A new source developed by Germany-based laser supplier Klastech is claimed to be the first ruby laser to achieve 100 mW of continuous-wave (CW) emission at 694.3 nm, by diode-pumped solid-state (DPSS) laser technology.
"This is the first CW ruby laser ever introduced to the market," Klastech founder and CEO Fedor Karpushko told optics.org. "We have already demonstrated an output power of 120 mW, and higher powers are being developed for the future."
This achievement is the first step towards an efficient CW UV laser source at 347 nm via generation of the second harmonic from ruby's fundamental emission.
The key to success lies in the company's use of two stages of intracavity power enhancement at the fundamental frequency, a technique termed double-enhanced intracavity frequency conversion (DENICAFC).
"In a conventional DPSS laser the power at the fundamental wavelength is fed through a nonlinear crystal to produce a shorter wavelength," explained Chris Madin, Klastech chief sales officer. "Both the laser active crystal and nonlinear crystal are placed within the same laser cavity."
The DENICAFC approach puts the second crystal inside its own cavity, impedance-matched with the active crystal cavity in a patented three-mirror arrangement. "This allows you to multiply the level of the fundamental frequency power interacting with the nonlinear crystal without upsetting the optimal lasing conditions for the laser active crystal," said Madin. "This massively increases the efficiency with which you produce the second harmonic – up to ten times higher than any other method."
DENICAFC brings with it a number of advantages. "It gives us the opportunity to produce a wide variety of wavelengths not otherwise possible in CW, and it allows us to generate shorter wavelengths without having to use very large pump lasers or pulsed technology," said Madin. "It produces locked single-line longitudinal mode output, with long coherence lengths and spectral line widths below 1 MHz, without the cost penalty that controlling those parameters would normally involve. That is very attractive for spectroscopy and industrial holography applications."
The new ruby laser itself is not the whole story. "There is a huge interest in producing CW light at around 350 nm," said Karpushko. "To date there are a few commercial systems available that attempt to do this through 'tripling' the frequency of IR light at 1064 nm, but this presents difficulties. Coherence must be maintained throughout, and the natural efficiency of the frequency tripling process is very low. In effect, it requires a large amount of original pump power to eventually produce some tens of milliwatts of output."
Karpushko's alternative proposal, currently pending patent approval, is to create UV at around 350 nm as the second harmonic of a ruby laser. "We use two stages of second harmonic generation to reach 347 nm, instead of tripling," he explained. "We take the output of a Nd-doped crystal laser and turn it into 532 nm green light by frequency-doubling using DENICAFC technology, and then use that to pump the ruby laser."
DENICAFC will again be employed to convert the ruby output from 694 to 347 nm. "You do not have to maintain phase-matching conditions and coherence between the first and second operations, as they are independent," noted Karpushko. "The overall process is far more efficient."
Klastech plans to unveil the new ruby laser source as a product in its own right at the LASER show in Munich in June 2009.
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