PARAMETRIC OSCILLATORS: Compact, high-power device emits at telecom wavelengths

Distributed-feedback lasers with power output in the tens of milliwatts have become preferred sources for many data-communications applications, but these lasers generally require erbium amplification to extend the output into the 100-mW range. A recently developed discrete, single-frequency optical parametric oscillator (OPO) may soon provide a simpler alternative, both between 1530 and 1580 nm for telecommunications and at longer or shorter wavelengths where scientific measurements and L-band applications are often performed but where amplification cannot be accomplished with erbium devices.

The new device, a 0.5-W, single-frequency, 1510- to 1630-nm, pump- and signal-resonant OPO, under development at Lightwave Electronics (Mountain View, CA), has been designed into a solid block of zero-thermal-expansion (Invar) steel with mirrors bonded around the outside and beam tubes drilled within. "I call it quasi-monolithic," said project leader Mark Arbore. "It has a lot of the beneficial qualities of a monolithic structure, but the design allows us a lot of freedom in optimizing."

A 1.5-W nonplanar ring oscillator (NPRO) pumps the device, which consists of a bowtie-shaped ring cavity with four mirrors (two flat and two with 50-mm radius of curvature) and a 2-cm-long crystal of periodically poled lithium niobate (PPLN). With the exception of 95% reflectivity on the input coupler at 1064 nm and 97% on the output coupler at 1550 nm, all of the mirrors were highly reflective at both 1064 and 1550 nm. Three of the mirrors were bonded firmly to the periphery of the device.

Adjustment of cavity resonance to the NPRO wavelength is accomplished through a fourth mirror (one of the flat ones) connected adjustably to the steel block through a 1-cm, temperature-controlled aluminum spacer that allows a relatively slow cavity-length adjustment on the order of 20 µm. Approximately 1 µm of rapid adjustment (up to 10 kHz) can be obtained by applying up to 200 V to piezoelectric-transducer elements bonded to the top and bottom of the device, which is useful for canceling out high-frequency noise from a shock or vibration, Arbore said.

"Normally when you control the length of the cavity in a locked situation, you mount one of the mirrors on a piezo and move the mirror relative to the cavity," he said. "But here, all of the mirrors are glued to the cavity, and then we stretch the cavity with the piezo."

The innovative cavity-locking method, for which the company has applied for a patent, essentially provided a vehicle for bringing together several existing technologies into a compact OPO that can operate in its efficient, above-threshold regime while also remaining relatively insensitive to temperature, shock, and vibration, Arbore said. "The PPLN has a high gain, and we're also resonating the pump light very strongly," he added. "So we have tens of watts of power circulating at 1064 nm in this resonant cavity, which is possible because of the single-frequency NPRO. Arbore's team is currently characterizing device prototypes in hopes of delivering a commercial product by next month.

"At this point, we're characterizing the performance," he said. "We know how much power it puts out, and we've made several hundred milliwatts in the laboratory. But there are more subtle specs that we are measuring now, such as understanding the noise and the details of how it tunes and how stable it is."

Beyond applications at 1550 nm that would take advantage of technology developed for the telecommunications industry, the researchers hope to produce an "ultrapure" stable-frequency OPO that will ultimately offer a variety of wavelengths from 1400 to 1900 nm for a range of scientific measurements.

"It may be overkill for some things," Arbore said. "But it could be the ultimate in high-power, narrow-linewidth sources for a lot of different people."