Long-external-cavity distributed Bragg reflector laser with subkilohertz intrinsic linewidth

Abstract

We report on a simple, compact, and robust 780 nm distributed Bragg reflector laser with subkilohertz intrinsic linewidth. An external cavity with optical path length of 3.6 m, implemented with an optical fiber, reduces the laser frequency noise by several orders of magnitude. At frequencies above 100 kHz the frequency noise spectral density is reduced by over 33 dB, resulting in an intrinsic Lorentzian linewidth of 300 Hz. The remaining lowfrequency noise is easily removed by stabilization to an external reference cavity. We further characterize the influence of feedback power and current variation on the intrinsic linewidth. In this Letter, we present a tunable, long-externalcavity DBR laser with a 3000-fold reduction of its intrinsic linewidth to 300 Hz. This is achieved by implementing a 3.6 m long external cavity using an optical fiber. The system exhibits noise suppression of over 33 dB at frequencies of 100 kHz and above. Low-frequency noise due to mechanical or thermal fluctuations in the feedback path can be controlled by stabilizing the laser to a reference cavity using an active servo loop. Feedback-induced mode instability [16] is avoided by operating within a 13 dB wide range of feedback power.We use a 780 nm DBR laser (Photodigm PH780DBR120T8-S) with 120 mW maximum output power as our source. The laser diode has a front facet reflectivity close to 1%, effective gain region length of 1.8 mm, and DBR reflectivity of 60% (all values are nominal values provided by manufacturer). The diode is temperature stabilized to slightly below room temperature. Throughout our measurements, we operated the laser at 100 mA (16 mW total output power). A beam splitter deflects 10% of the laser power into a 2 m long polarization-maintaining optical fiber that constitutes the feedback path (Fig. 1). An aperture is added before the angled fiber to block unwanted backreflection from the fiber tip. A mirror mounted on a piezoelectric transducer (PZT) reflects the light back into the fiber, and a quarterwave plate in combination with a polarizing beam splitter is used to adjust the feedback power. A maximum −30 dB fractional power can be reflected back into the laser. (Henceforth we denote the fractional feedback power incident on the laser collimator as p, which may be less than the fractional power that is mode matched into the active region of the laser diode.) The externalcavity laser is set up on a 12 00 × 18 00 aluminum baseplate and enclosed by a plastic case to reduce environmental perturbations.In general, the effect of external cavity optical feedback depends on the feedback power, cavity length, and the phase of the feedback light, as summarized by Tkach Fig. 1. Schematic setup for the long-external-cavity laser and the frequency noise characterization. M, mirror; AP, aperture; PM fiber, polarization-maintaining fiber; QWP, quarter-wave plate; BS, beam splitter; PBS, polarization beam splitter; APD, avalanche photodiode; PZT, piezoelectric transducer.

Passive intrinsic-linewidth narrowing of ultraviolet extended-cavity diode laser by weak optical feedback

“…3 shows the Lorentzian linewidth as a function of the feedback power ratio, for both fiber lengths. The narrowing capability is confirmed to be better for a longer cavity, in agreement with theoretical expectations [19] and previous measurements on DBR lasers [9]. At low and intermediate feedback levels, we observe that for equal feedback ratios the linewidths follow the expected square law relation with feedback cavity length, wherein a ratio in the cavity length of ∼ 3 leads to a linewidth lower by almost an order of magnitude.…”

Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits

“…Much smaller bandwidths have been obtained with non-integrated lasers that use bulk optical gratings [69]. Miniaturized bulk components have been very effective as well [70], particularly high-Q whispering gallery mode resonators [71] or Bragg fibers [62,72,73]. In connection with extensive electronic servo stabilization, for research purposes, even diode lasers with bulk optical feedback can reach the sub-Hz-range [74,75,76].…”

“…Long-term frequency stability requires that the laser is frequency tunable, such that an electronic servo control can minimize the detuning from a stable reference used as frequency discriminator [60]. However, to avoid that such active stabilization adds too much noise on its own, e.g., quantum noise from photo detection in the frequency discriminator, and to suppress noise also at higher noise frequencies, it remains essential to reduce the intrinsic laser linewidth [61,62].…”

The water vapour self-continuum absorption in the infrared atmospheric windows: new laser measurements near 3.3 and 2.0 µm

“…A distributed feed back (DFB) diode laser (from Eblana Photonics) covering the 4995-5007 cm −1 spectral interval was used as light source. The optical feedback technique (Lin et al, 2012) was used to narrow the laser emission line width and increase the light injection into the cavity during resonances with cavity modes. The frequency of the laser diode was measured with a wavelength metre (model 621-A IR from Bristol Instruments, 8 MHz 1σ accuracy and 2.5 Hz acquisition rate).…”