Self-referencing a continuous-wave laser with electro-optic modulation

We phase-coherently measure the frequency of continuous-wave (CW) laser light by use of optical-phase modulation and f-2f nonlinear interferometry. Periodic electro-optic modulation (EOM) transforms the CW laser into a continuous train of picosecond optical pulses. Subsequent nonlinear-fiber broadening of this EOM frequency comb produces a supercontinuum with 160 THz of bandwidth. A critical intermediate step is optical filtering of the EOM comb to reduce electronic-noise-induced decoherence of the supercontinuum. Applying f-2f self-referencing with the supercontinuum yields the carrier-envelope offset frequency of the EOM comb, which is precisely the difference of the CW laser frequency and an exact integer multiple of the EOM pulse repetition rate. Here we demonstrate absolute optical frequency metrology and synthesis applications of the self-referenced CW laser with <5E-14 fractional accuracy and stability.Comment: 8 pages, 4 figure

Carrier-Envelope Offset Stabilized Ultrafast Diode-Pumped Solid-State Lasers

Optical frequency combs have been revolutionizing many research areas and are finding a growing number of real-world applications. While initially dominated by Ti:Sapphire and fiber lasers, optical frequency combs from modelocked diode-pumped solid-state lasers (DPSSLs) have become an attractive alternative with state-of-the-art performance. In this article, we review the main achievements in ultrafast DPSSLs for frequency combs. We present the current status of carrier-envelope offset (CEO) frequency-stabilized DPSSLs based on various approaches and operating in different wavelength regimes. Feedback to the pump current provides a reliable scheme for frequency comb CEO stabilization, but also other methods with faster feedback not limited by the lifetime of the gain material have been applied. Pumping DPSSLs with high power multi-transverse-mode diodes enabled a new class of high power oscillators and gigahertz repetition rate lasers, which were initially not believed to be suitable for CEO stabilization due to the pump noise. However, this challenge has been overcome, and recently both high power and gigahertz DPSSL combs have been demonstrated. Thin disk lasers have demonstrated the highest pulse energy and average power emitted from any ultrafast oscillator and present a high potential for the future generation of stabilized frequency combs with hundreds of watts average output power

Low Noise, High Repetition Rate Semiconductor-based Mode-locked Lasers For Signal Processing And Coherent Communications

This dissertation details work on high repetition rate semiconductor mode-locked lasers. The qualities of stable pulse trains and stable optical frequency content are the focus of the work performed. First, applications of such lasers are reviewed with particular attention to applications only realizable with laser performance such as presented in this dissertation. Sources of timing jitter are also reviewed, as are techniques by which the timing jitter of a 10 GHz optical pulse train may be measured. Experimental results begin with an exploration of the consequences on the timing and amplitude jitter of the phase noise of an RF source used for mode-locking. These results lead to an ultralow timing jitter source, with 30 fs of timing jitter (1 Hz to 5 GHz, extrapolated). The focus of the work then shifts to generating a stabilized optical frequency comb. The first technique to generating the frequency comb is through optical injection. It is shown that not only can injection locking stabilize a mode-locked laser to the injection seed, but linewidth narrowing, timing jitter reduction and suppression of superfluous optical supermodes of a harmonically mode-locked laser also result. A scheme by which optical injection locking can be maintained long term is also proposed. Results on using an intracavity etalon for supermode suppression and optical frequency stabilization then follow. An etalon-based actively mode-locked laser is shown to have a timing jitter of only 20 fs (1Hz-5 GHz, extrapolated), optical linewidths below 10 kHz and optical frequency instabilities less than 400 kHz. By adding dispersion compensating fiber, the optical spectrum was broadened to 2 THz and 800 fs duration pulses were obtained. By using the etalon-based actively mode-locked laser as a basis, a completely self-contained frequency stabilized coupled optoelectronic oscillator was built and characterized. By simultaneously stabilizing the optical frequencies and the pulse repetition rate to the etalon, a 10 GHz comb source centered at 1550 nm was realized. This system maintains the high quality performance of the actively mode-locked laser while significantly reducing the size weight and power consumption of the system. This system also has the potential for outperforming the actively mode-locked laser by increasing the finesse and stability of the intracavity etalon. The final chapter of this dissertation outlines the future work on the etalon-based coupled optoelectronic oscillator, including the incorporation of a higher finesse, more stable etalon and active phase noise suppression of the RF signal. Two appendices give details on phase noise measurements that incorporate carrier suppression and the noise model for the coupled optoelectronic oscillator

High-precision optical and microwave signal synthesis and distribution

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 135-148).In this thesis, techniques for high-precision synthesis of optical and microwave signals and their distribution to remote locations are presented. The first topic is ultrafast optical pulse synthesis by coherent superposition of mode-locked lasers. Timing and phase synchronization of ultrabroadband Ti:sapphire and Cr:forsterite mode-locked lasers is studied. Subfemtosecond (1 h) 3-mrad level phase stability of a 10.225 GHz microwave signal extracted from a mode-locked laser is demonstrated. The third topic is timing stabilized fiber links for large-scale timing distribution. Precise optical timing distribution to remote locations can result in synchronization over long distances. In doing so, acoustic noise and thermal drifts introduced to the fiber links must be canceled by a length-correction feedback loop. A single type-II phase-matched PPKTP crystal is used to construct a compact and self-aligned balanced optical cross-correlator for precise timing detection.(cont.) Using this correlator, a 310 m long fiber link is stabilized with long-term sub-10 fs accuracy. The final topic is photonic analog-to-digital conversion of high-frequency microwave signals. Sampling of high-frequency (>10 GHz) microwave signals is challenging due to the required aperture jitter below 100 fs. An optical subsampling down- converter for analog-to-digital conversion of narrowband high-frequency microwave signals is studied. The measured signal to noise-and-distortion ratio of 1-Mbps signals at 9.5 GHz carrier frequency is 22 dB over 2 MHz bandwidth. By integrating the demonstrated techniques, large-scale femtosecond-precision timing distribution and synchronization systems can be implemented.by Jungwon Kim.Ph.D

Cr: Colquiriite Lasers: Current Status and Challenges for Further Progress

Cr: Colquiriite laser materials (Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF) own broad absorption bands in the visible region that allow direct-diode pumping by well-developed low-cost red diodes. Moreover, they possess broad emission bands in the near infrared that enable widely tunable laser operation (720–1110 nm), and generation of sub-10-fs light pulses via mode-locking. Furthermore, Cr: Colquiriite crystals can be grown with a very low loss level of 0.2%/cm, which enables the construction of high-Q-cavities, resulting in lasing thresholds below 1 mW, and slope efficiencies above 50%. High-Q-cavities constructed with Cr: Colquiriites could store large amount of intracavity laser powers which is off great interest: (i) for increasing the efficiency of intracavity nonlinear processes such as intracavity frequency-doubling, and (ii) for minimizing laser noise such as timing jitter noise in femtosecond operation. However, thermally and mechanically Cr: Colquiriites have glass like properties. Hence, average power scaling has been challenging in the cw and femtosecond Cr: Colquiriite lasers, as well as in their amplifiers. In this paper, we will review research efforts over the last decades, in developing robust, low-cost, highly-efficient, and tunable cw and femtosecond laser sources based on diode-pumped Cr:Colquiriite gain media. Challenges for future progress will also be discussed.No sponso

Sub-femtosecond precision timing distribution, synchronization and coherent synthesis of ultrafast lasers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 180-189).In this thesis, we present a complete set of techniques for sub-femtosecond measurement, control and distribution of ultrafast optical pulse trains, with respect to pulse timing and phase. First, analytical analysis of the balanced optical cross-correlator (BOC) for attosecond precision pulse timing measurement is presented for both short and long crystal devices. It is found that the sensitivity of the long crystal BOC is independent of pulse duration, to first order. In addition, analytical noise models predict 13 as rms resolution, within a 1 MHz bandwidth, for optical pulses consistent with a practical fiber optic timing link. This analysis aids the widespread adoption of the BOC technique for other wavelengths and implementations. Secondly, long term timing distribution of a 200 MHz ultrafast optical pulse train over 340 m of single mode optical fiber is demonstrated, using the BOC. In this way, the group delay of the fiber link is directly stabilized with unprecedented precision and longterm stability. In addition, by distributing the entire optical pulse train, all optical and RF harmonics are provided at the remote location for direct synchronization of remote ultrafast lasers and microwave electronics. Over 168 hours of continuous, unaided operation, a drift of 5 fs rms is achieved, with less than 1.5 fs rms drift at timescales up to 10,000 seconds. Additional analysis of factors effecting performance, such as polarization mode dispersion and fiber nonlinearity is studied through experiment and simulations. It is found that nonlinear-origin drifts can be avoided for pulse energies below 40 pJ. A chirped pulse method could be implemented to distribute pulses of higher energy. Thirdly, the first quantum-resolution timing jitter measurement of ultrafast laser timing jitter for passively mode-locked lasers up to the Nyquist frequency is presented. The total jitter from for a 79.4 MHz stretched pulse erbium fiber laser is found to be 2.6 fs rms [10 kHz, 39.7 MHz]. It is found that the timing jitter power spectral density scales with frequency according to that expected for a white noise source, in agreement with theory. However, unexpected spurious jitter at high frequencies can occur for some mode-locked states, adding up to 5.5 fs rms jitter. Similar measurements of a 200 MHz erbium fiber soliton laser reveal the decay time of center frequency fluctuations to be 17 ns, with a predicted excess noise of approximately ten. These measurements suggest that timing jitter can be decreased through improved amplifier design. Finally, the synchronization of a 8 fs fiber supercontinuum at 1200 nm to a 7 fs Ti:Sapphire laser pulse train at 800 nm is achieved for both pulse timing and phase with attosecond precision. This achievement is enabled by the development of a novel scheme for stabilization of the carrier envelope offset of the entire optical bandwidth of an octave spanning supercontinuum, without introducing excess timing jitter. In particular, by implementing an acousto-optic frequency shifting (AOFS) feedback system within a fiber supercontinuum source, carrier envelope phase locking, to the Ti:Sapphire laser, is demonstrated to within 200 mrad rms [100 Hz, 5 MHz]. Previous techniques lack the high-speed, orthogonal control of CEP and pulse timing and broad optical bandwidth for synthesizing few-cycle optical pulses. Furthermore, timing synchronization of 280 as rms is achieved through combined piezoelectric and electro-optic feedback on the fiber supercontinuum, as measured with the BOC. This work enables the synthesis of a frequency comb spanning 650 to 1400 nm, resulting in a 3.5 fs transform limited pulse duration-assuming ideal spectral phase compression. To date, the spectrum has been successfully compressed to 4.7 fs, as measured with two-dimensional spectral shearing interferometry (2DSI). Moreover, by stabilizing a fiber supercontinuum source to a low-noise Ti:Sapphire laser, the ultra-high stability of the Ti:Sapphire laser is fully transferred to the octave spanning supercontinuum.by Jonathan A. Cox.Ph.D

Ultra-Low Phase Noise Microwaves from Optical Signals

Continuous-wave lasers locked to high-finesse optical reference cavities are oscillators that produce ~500 THz optical signals with unprecedented stability. Indeed, sub-femtosecond fractional frequency instability at one second averaging can now be achieved. A self-referenced femtosecond laser frequency comb (FLFC) is used as a frequency divider to provide a phase-coherent link between optical and microwave domains, dividing the frequency down to the gigahertz range while also transferring the stability of the original signal. Photodetectors then convert the optical pulses into electronic signals. The resultant 10 GHz microwave signals have ultra-low phase noise below -100 dBc/Hz at 1 Hz offset, surpassing that of traditional microwave oscillators. This new approach offers significant improvement for many applications that rely on stable microwave signals, and may even create new measurement technologies otherwise unachievable with current signal sources. In reality, fundamental and technical sources of noise in each stage of the optical-to-microwave generation process limit the ultimate achievable stability of the signal. Optical reference cavities are limited by environmental effects and thermal fluctuations, and FLFC dividers suffer from intrinsic timing jitter, amplitude noise, and limited stabilization servo bandwidth. However, it is the seemingly straightforward photodetection of optical pulses that proves to be the limiting factor in the ultimate noise floor of these signals. In this thesis, I describe the noise limitations of each part of the optical-to-microwave scheme, particularly focusing on the noise limitations of photodetection. I will give a basic representation of these photodetection noise phenomena in terms of the physical behavior of optically-generated electrons in semiconductor photodiodes. The two main photodetection noise phenomena--shot noise and amplitude-to-phase conversion--will be thoroughly characterized in the context of generation of 10 GHz low phase noise signals. Finally, I will use this characterization of photodetector noise to choose optimal photodetectors and operating conditions to realize unprecedentedly low phase noise signals with a variety of optical-to-microwave generation schemes