Title in 18-Point Times Font
Total Page:16
File Type:pdf, Size:1020Kb
College of Engineering Drexel E-Repository and Archive (iDEA) http://idea.library.drexel.edu/ Drexel University Libraries www.library.drexel.edu The following item is made available as a courtesy to scholars by the author(s) and Drexel University Library and may contain materials and content, including computer code and tags, artwork, text, graphics, images, and illustrations (Material) which may be protected by copyright law. Unless otherwise noted, the Material is made available for non profit and educational purposes, such as research, teaching and private study. For these limited purposes, you may reproduce (print, download or make copies) the Material without prior permission. All copies must include any copyright notice originally included with the Material. You must seek permission from the authors or copyright owners for all uses that are not allowed by fair use and other provisions of the U.S. Copyright Law. The responsibility for making an independent legal assessment and securing any necessary permission rests with persons desiring to reproduce or use the Material. Please direct questions to [email protected] Fundamental Mode-Locking of a Composite-Cavity Electro-optic Microchip Laser for Microwave Photonics David Yoo, Asher Madjar, Samuel Goldwasser, and Peter Herczfeld Center for Microwave/Lightwave Engineering, ECE Department, Drexel University, Philadelphia, PA, 19104, USA E-mail: [email protected] Abstract — Compact solid-state lasers are an attractive mode-locking be fully explored. This is the emphasis of potential source for high speed, low noise optical pulses the work presented here. suitable for microwave photonic systems such as optical A/D conversion or digital communications. These small devices can be mode-locked at the fundamental cavity II. FUNDAMENTAL VS. HARMONIC MODE-LOCKING resonance, thereby avoiding the complications arising from the presence of supermodes in harmonically mode-locked Fundamental mode-locking is typically established by lasers. An Nd:YVO4/MgO:LiNbO3 microchip laser was modulating either the optical loss or phase inside a laser mode-locked at a fundamental frequency of 20 GHz. When with an electronic oscillator close to the cavity locked to a synthesizer, the phase noise was source-limited to -72 dBc/Hz at 1 kHz offset. A coupled optoelectronic resonance. This leads to a pulsed optical output at a rate oscillator structure was then used to mode-lock the device equal to this resonance frequency. In order to achieve without a microwave source, yielding a further reduced the picosecond pulse rates desirable for high speed phase noise of -95 dBc/Hz at 1 kHz offset. sampling, the laser cavity resonance frequency must be Index Terms — Mode locked laser, analog-digital in the microwave range. For solid-state lasers, this conversion, optical communication, solid lasers, electrooptic translates into millimeter-scale devices. devices, timing jitter. Such a scale is impractical for fiber lasers. Instead, they are harmonically mode-locked to generate high I. INTRODUCTION speed optical pulses. However, harmonic mode-locking can also support multiple supermodes inside the cavity. For high bandwidth digital or analog/digital mixed These supermodes are not correlated, and the noises of signal systems, there are applications where photonic neighboring wavelengths belonging to differing replacement of electronic components is desirable. New supermodes do not completely cancel at the digital microwave photonic sub-systems (such as optical photodetector. This is observed as noise spurs at offset ADCs) require stable, rapid optical pulses for samplers frequencies corresponding to the harmonics. or clocks. In these scenarios, mode-locking is the In practice, the phase noise of mode-locked pulses is architecture of choice. usually dominated by the locking source. Since the Much literature has recently been presented on the timing jitter is the integral of the phase noise, low noise design and application of mode-locked EDFA ring lasers is important for stable pulses, and the integration limits as a higher quality alternative to semiconductor lasers. of the phase noise are an essential consideration. From This technology is well-established and highly modular sampling theory, it is reasonable to require low phase in nature. However, their high number of components, noise at frequencies up to the Nyquist (i.e., half the pulse cost, and large footprints make fiber lasers impractical repetition) frequency to fully exploit the bandwidth. For for many applications. More importantly, their long pulse rates at tens of GHz, this means that phase noise is lengths translate into low cavity resonances, usually in still important at offset frequencies where harmonically the MHz range. For rapid pulse rates, these lasers must mode-locked lasers can particularly suffer from be mode-locked at a high harmonic of the fundamental thousands of spurs which will be cumulatively resonance. This harmonic mode-locking can generate incorporated as additional timing jitter. supermode noise spurs. Although suppression schemes Fundamental mode-locking of solid-state lasers is a have been investigated [1], such methods require even desirable alternative for high sampling rates. However, more components. due to the typically shorter cavity lengths, there are In contrast to harmonic mode-locking, fundamental several issues which must be considered that are not mode-locking of solid-state lasers is relatively less present (or have reduced importance) in lasers studied for microwave photonic applications (due to employing harmonic mode-locking. These include: factors such as the difficulty realizing a suitable laser), • Modal competition effects in short-cavity lasers yet is immune to supermode noise. For low noise, high • Optimization of microwave/optical wave interaction bandwidth operation, it is thus essential that fundamental An important sacrifice when switching from harmonic Using a Fabry-Perot architecture with a to fundamental mode-locking is the loss of the ability to homogeneously broadened gain material (such as easily add more components inside the laser to alter Nd:YVO4) reduces the potential number of free-running functionality at any given time. As a result, though lasing modes. This makes the predictability of the optical lasers which employ fundamental mode-locking may behavior (especially modal competition) an important appear to have deceptively simple structures, functional consideration for regenerative mode-locking. Therefore, considerations must be accounted for in their entirety spatial hole burning effects were investigated to ensure before device assembly. that a laser cavity designed to have a free spectral range of 20 GHz did in fact have multimode operation of adjacent cavity modes. This is needed to yield the free- III. LASER DESIGN FOR FUNDAMENTAL MODE-LOCKING running microwave signal at the fundamental cavity resonance necessary for feedback. A. Laser Configuration A set of experiments were devised to investigate the suitability of fundamental mode-locking in a composite- a cavity solid-state laser for microwave photonic applications. An Nd:YVO4/MgO:LiNbO3 electro-optic λ ω microchip laser was selected in order to achieve high 2×FSR 2×FSR optical spectral quality at 1.06 µm while maintaining a small form factor [2]. These materials are commonly available, and are no more difficult to assemble (or mass b produce) than the common green laser pointer. Since λ ω fundamental mode-locking is generally impractical at FSR FSR microwave frequencies inside a ring architecture, a Fabry-Perot structure was employed. The use of discrete Fig. 1. Conceptual diagrams of modal competition issues in gain and modulator crystals also allowed a degree of homogeneously broadened materials. Spectra on left represent modularization by offering the possibility of moving to the free-running optical signals; spectra on right are the different operating wavelengths (e.g., 1.34 or 1.55 µm) detected microwaves. Cases (a) and (b) are discussed below. by simply switching crystals. The intracavity lithium niobate phase modulator had a A simulation was conducted to determine regions of length of approximately 1.75 mm, while the vanadate high correlation of the standing waves of potential gain section had a length of 0.5 mm. Dielectric coatings longitudinal modes inside the laser. An experiment to were deposited on all optical surfaces, with 98% verify the conclusions of the simulation was also carried reflectivity on the far surface of the modulator and out, by suspending an antireflection-coated Nd:YVO4 antireflection coatings on the others. A glass mirror with crystal between two mirrors. This formed a laser cavity a high reflectivity to the lasing wavelength (1064 nm) where the position of the gain crystal relative to the two and low reflectivity to the pump wavelength (808 nm) mirrors could be adjusted with a translation stage. was used to complete the optical cavity. The distance Both the simulation and experiment confirmed that between the two optical mirrors was adjusted to create a placement of the gain medium was a critical design laser cavity with a free spectral range near 20 GHz. The factor significantly affecting the free-running optical input pump beam was provided by a narrow-stripe laser performance. Having a configuration where a mirror diode, using free space coupling optics to minimize the was directly deposited on (or closer than 2.5 mm to) the beam waist inside the gain crystal in order to enforce gain medium