Diode laser produces low-noise chirped pulses

Lasers with ultrashort pulses and repetition rates on the order of 100MHz using solid-state or fiber gain media are very common, since they are easily fabricated using meter-long laser resonators. However, a variety of applications call for laser sources of uniform intensity and exceptionally chirped pulses characterized by low pulse-to-pulse energy fluctuations. Continuous-operation photonic time-stretched analog-to-digital conversion (TS ADC), an ADC implementation that exceeds conventional ADC speeds, requires a low-noise, uniform, linearly chirped pulse train that fills the time slots between the pulses.¹ In addition, some implementations of optical coherence tomography (OCT), an in vivo medical-imaging technique, also make use of uniform-intensity and linearly chirped laser sources at repetition rates in the MHz regime.²

The TS ADC experiments that have been demonstrated used stretched pulses originating from narrow-pulse lasers, while OCT laser sources included a mechanically swept optical filter.^1,2 Although both approaches provide linearly swept pulses, they do not address the problem of requiring quasi-continuous-wave (CW), uniform-intensity pulses that cover the laser period with low pulse-to-pulse energy fluctuations at 100MHz repetition rates. Moreover, while semiconductor-diode lasers tend to operate at multi-GHz repetition rates, they are advantageous for field applications, since they have small footprints, are rugged, temperature stable, power efficient, and inexpensive. Here we focus on an unconventional laser-cavity design, the Theta laser, which enables use of semiconductor gain media in long cavities characterized by low repetition rates and noise.³

Our Theta laser-cavity design employs extreme chirped-pulse amplification inside the cavity.³ The optical pulses stretch and recompress on every roundtrip inside the laser, using the opposite ports of a single chirped fiber-Bragg grating (see Figure 1). In steady state, the pulses pass through the gain medium fully stretched, filling the laser period as a uniform-intensity quasi-CW frequency-chirped pulse train. The Theta laser is an actively and harmonically mode-locked external-cavity laser. Harmonic operation of mode-locked lasers leads to noisy laser performance because of the limited correlation between intracavity pulses, which is demonstrated as supermode noise spurs (SNS) in the radio-frequency spectra of the photodetected pulse train. A Fabry-Perot etalon with a free spectral range (FSR) equal to the laser's repetition rate stores and mixes the pulses, thus increasing their homogeneity and reducing laser noise.⁴ We used a fiberized etalon, which is affected by FSR drift and susceptible to acoustic and mechanical vibrations. Nonetheless, we used the birefringence of the fiberized etalon in an intracavity Hänsch-Couillaud scheme to generate an error signal for referencing the laser to the etalon.⁴

Figure 1. Schematic of the Theta laser with an intracavity etalon and long-term referencing. BPF: Optical band-pass filter. CFBG: Chirped fiber-Bragg grating. CIRC: Optical circulator. DCF: Dispersion-compensating fiber. EOM: Electro-optic intensity modulator. FS: Fiber stretcher. IM: Intermixer. OC: Output coupler. PBS: Polarization beam splitter. PC: Polarization controller. PD: Photodetector. POL: Polarizer. SOA: Semiconductor optical amplifier.⁴

The Theta laser (see Figure 2) has a repetition rate of 99.580MHz. Pulses from its compressed-port output are measured in 10ns periods and are 30ps long, resulting in a duty cycle of <0.3%. Note that duty cycles in this range cannot be attained using conventional linear or ring-cavity designs for semiconductor-based lasers at these repetition rates. The etalon's storage and mixing function suppresses the SNS by >10dB in the radio-frequency spectra. Careful dispersion compensation of the ≈100m-long laser cavity enables lasing at the full supported bandwidth, even with the use of a Fabry-Perot etalon (bandwidth 10nm). The pulses are linearly chirped and exhibit a quasi-CW time-intensity profile.

Figure 2. Photograph of the Theta laser. A double-walled box is used for acoustic insulation.

These applications of the Theta laser impose stringent requirements on the pulse-energy fluctuations. We observed a significant improvement in laser-amplitude noise performance upon insertion of the etalon and referencing the laser cavity to the etalon. We measured a 20-fold improvement, from 1 to 0.05%, in the frequency offset range containing the SNS, [1, 20]MHz. Thus, use of the fiberized etalon offers major advantages and enables applications that require low noise.

While the progress in OCT and photonic TS ADC attained using conventional, frequency-chirped laser sources is promising, time-filling, low-noise, and high-repetition-rate laser pulses are required for further progress. Our results from the Theta laser show linearly chirped pulses that fill the laser period with ultralow-amplitude noise. Our next step will be to use a more stable Fabry-Perot etalon as reference for the laser cavity and improve the noise characteristics of the driving pulsed electrical signal.

We acknowledge support from the National Science Foundation and the Defense Advanced Research Projects Agency.