Minilaser Is Tunable, Single-Frequency Source at 1.5 µm

Breck Hitz

A widely tunable, single-frequency laser operating in the 1.5-µm spectral region has application not only in testing and qualifying C-band telecom components, but also in Doppler-shifted velocity measurements and in spectroscopic studies. A group of scientists affiliated with the Royal Institute of Technology and Acreo AB, both in Stockholm, Sweden, and with institutes of the Consiglio Nazionale delle Ricerche in Milan, Italy, has developed a cost-effective approach for such a device based on an Er-Yb:glass minilaser.

Figure 1. A new tunable single-frequency laser for the C-band features an Er-Yb:glass minilaser and a mechanically deformable fiber Bragg grating. The minilaser consists of a 1-mm-thick flat-flat Er-Yb:glass disk with a laser mirror on its back side, and a spherical output mirror. The front side of the glass disk is antireflection-coated at the laser wavelength, and the back is transmissive to the 975-nm diode-laser pump radiation.

Tunable single-frequency oscillation is achieved by coupling the minilaser to an external resonator that comprises approximately 14 m of single-mode fiber and a mechanically deformable fiber Bragg grating (Figure 1). The 7-mm-long Bragg grating is encased in snug-fitting ceramic ferrules and compressed with a micrometer screw (Figure 2). The researchers found that stretching the grating led to breakage before the desired tuning could be achieved but that compressing it enabled adequate tuning without breakage.

Figure 2. By compressing the fiber Bragg grating, the laser can be tuned from 1530 to 1560 nm.

When the minilaser was run alone, it oscillated in multiple longitudinal modes, but its M² of 1.05 indicated near-single-transverse-mode oscillation. When coupled with the external fiber Bragg grating and when the resonator frequencies were matched by adjusting the piezoelectric transducer on the minilaser's output mirror, the laser's spectrum narrowed so much that the group's 60-MHz-resolution Fabry-Perot interferometer could not measure it. Other measurements indicated that the bandwidth may have been as narrow as 10 kHz.

The laser's output power, emerging through the fiber Bragg grating, was within a few milliwatts of 40 mW as the laser was step-tuned from 1530 to 1560 nm across the C-band. Step tuning occurred as the peak of the minilaser (whose free spectral range was 17 GHz) moved from one peak of the fiber Bragg grating resonator (whose free spectral range was 7 MHz) to the next. Output could be obtained at any frequency by simultaneously adjusting the piezoelectric transducer and the grating, but to tune the laser across the C-band, the scientists adjusted only the grating.

Although the laser oscillated most of the time on a single longitudinal mode, they occasionally observed a beat note, indicating mode hopping between adjacent modes of the minilaser. Mode hopping and frequency drift occurred frequently in the laser as a result of thermal drift and the corresponding changes in length of the 14-m fiber between the Bragg grating and the mini-laser.

To eliminate the problem, the researchers shortened the fiber to 5 cm, removed the laser-to-fiber coupler and coupled the laser to the fiber using a simple aspheric lens. To eliminate polarization instabilities, they inserted an antireflection-coated polarizing prism between the minilaser and the fiber, and they also improved the configuration of the pump optics.

In this arrangement, the minilaser alone produced 190 mW of multimode output when pumped with 750 mW (compared with 150 mW under similar pumping in the previous configuration). When coupled with the external resonator, the bandwidth dropped to less than the measuring Fabry-Perot's resolution of 60 MHz, and no mode hopping was observed.

This new configuration has not been frequency tuned, but the scientists expect that it will produce approximately 85 mW across the C-band. Increasing the output coupling from the laser and the coupling efficiency between the minilaser and the single-mode fiber could yield further improvements, and the laser could be pumped harder without ill effect. By implementing all these techniques, the laser could be scaled to 150 mW of tunable, single-frequency output with an electrical efficiency as great as 9 percent, they believe.