Tunable laser

A tunable laser is a laser whose wavelength can be set, by a control input, anywhere within a stated range. Every laser tunes a little: a DFB laser moves by about 0.09 nm/K with temperature at 1550 nm (wavelength temperature coefficient), or about 3 nm over 35 K. The name usually means more than that, a range of tens of nanometers or more, set electronically or mechanically, with the laser running on a single mode throughout. What a tunable laser can do depends on three things: a gain medium whose bandwidth covers the range, a wavelength filter inside the cavity that can be moved across it, and a way of keeping one longitudinal mode of the cavity on the filter's peak as the filter moves.

Filter and cavity modes

The filter chooses the wavelength, but the laser can oscillate only on a cavity mode, a wavelength for which a whole number of half-wavelengths fits the optical length of the cavity. When the filter moves and the cavity does not, the lasing wavelength follows the filter in steps: it stays on one mode until a neighbor lies closer to the filter peak, then hops by one mode spacing. Tuning without hops requires the optical length to change in proportion to the wavelength, so that the number of half-wavelengths stays the same. In the external cavity laser of 23.6 mm optical length described in that entry, about 30,450 half-wavelengths fit at 1550 nm. Tuning it from 1520 to 1570 nm on one mode requires the optical length to grow by 0.76 mm, about 3%, in step with the wavelength; with the length held fixed, the laser would hop through about 990 modes, 6.35 GHz apart, on the way.

Tuning schemes fall into three kinds, following the classification of Buus, Amann and Blumenthal:

• Continuous (mode-hop-free) tuning keeps one mode across the range, so the frequency passes smoothly through every value. Spectroscopy of narrow lines and swept measurements of narrow resonances need it.
• Quasi-continuous tuning reaches every wavelength in the range as a set of overlapping continuous segments, with a hop from one segment to the next. It suits a laser that is set to a wavelength and held there, as on a WDM channel.
• Discontinuous tuning reaches only the wavelengths of the cavity modes nearest the filter peak, in a staircase with gaps between the steps.

Types of tunable laser

Monolithic semiconductor lasers. The DBR laser entry works through an example: lowering the index of the Bragg grating by 0.01 moves its reflection peak by 4.3 nm, but a cavity mode follows the peak continuously for only 1.2 nm unless the phase section can supply a larger index change, so a three-section laser tunes quasi-continuously. Sampled-grating lasers extend the range with the Vernier effect between two mirrors that reflect combs of slightly different spacing (Jayaraman, Chuang and Coldren), and superstructure gratings have tuned a DBR laser over more than 100 nm (Tohmori and colleagues). These lasers have no moving parts, and current tuning is fast, since the carrier density follows the current within about a nanosecond; the heat that the current deposits then shifts the wavelength slowly as the chip settles, which the control electronics correct. The currents for each channel are measured for each laser and stored in a table.

DFB arrays. A DFB array places about a dozen DFB lasers, 3 nm apart in wavelength, on one chip. Choosing a laser selects a 3 nm range and its temperature selects the wavelength within it, so every setting is a DFB laser running continuously on its one mode. The light of whichever laser is on reaches the fiber either through an MMI combiner, which loses 10.8 dB with 12 lasers and is followed by an amplifier on the chip, or through a MEMS mirror that steers the chosen beam into the fiber.

External cavity lasers. An external cavity laser closes a semiconductor gain chip with a diffraction grating, or with ring filters on a separate silicon or silicon nitride chip. The long cavity narrows the linewidth to kilohertz levels, against 1 to 10 MHz for a DFB laser, and a grating turned about the correct pivot keeps the cavity length in proportion to the wavelength (Liu and Littman), so the laser tunes continuously over tens of nanometers. Mechanical tuning is slower than current tuning, and the cavity length has to be held to a fraction of a nanometer against heat and vibration.

MEMS-tunable VCSELs. A VCSEL whose top mirror is a membrane suspended above the chip on a MEMS actuator tunes by moving that mirror, which changes the length of the cavity itself, so the one lasing mode moves with it. For a single mode to be tuned across 150 nm without a neighbor taking over, the modes must be spaced about that far apart, which at 1310 nm means an optical cavity length of no more than 5.7 µm, ${\lambda }^2\mathrm{/}(2\mathrm{\Delta }\lambda )$. A 1310 nm MEMS-VCSEL built this way has tuned continuously over 150 nm (Jayaraman and colleagues, 2012), and the light membrane can be swept at high repetition rates, as swept-source OCT requires. Fourier-domain mode-locked lasers, in which a tunable filter in a long fiber ring is driven in step with the round trip of the light, are another swept source for OCT (Huber and colleagues).

Broadly tunable lasers. Ti:sapphire has one of the broadest gain bands of any laser crystal and tunes from about 650 to 1100 nm (Moulton); single-frequency continuous-wave versions select their mode with an intracavity birefringent filter and etalon. Dye lasers cover 30 to 80 nm with each dye and were the main tunable lasers of the visible until solid-state sources took over most of their work. An OPO has no gain transition to fix its wavelength: phase matching picks the signal and idler, and with a 1064 nm pump and periodically poled lithium niobate the signal and idler tune between about 1.4 and 4 µm. In the mid-infrared itself, a quantum cascade gain chip in an external grating cavity has tuned from 7.6 to 11.4 µm (Hugi and colleagues).

Linewidth and wavelength control

Linewidth depends more on the length of the cavity than on the way it is tuned. External cavity lasers reach kilohertz linewidths and below; monolithic tunable lasers, whose cavities are at most a few millimeters long, have linewidths closer to those of DFB lasers, and injecting current into their tuning sections broadens the line further, because random fluctuations of the carrier density there make the refractive index fluctuate (Amann and Schimpe).

The wavelength of a semiconductor tunable laser also depends on its temperature and, through heating, on every current. A tunable transmitter therefore holds its channel with a wavelength locker, an etalon whose transmission changes steeply with frequency, and a thermoelectric cooler. The channels of WDM systems follow the DWDM grid of ITU-T G.694.1, anchored at 193.1 THz, with fixed spacings such as 50 and 100 GHz and a flexible grid. In the laboratory the wavelength is read on a wavemeter or referenced to gas absorption lines.

Uses

• WDM transmitters. One laser that reaches every channel replaces a separate part for each channel, in transmitter designs, in inventory and as spares, and lets a network assign wavelengths in operation, as in ROADM networks and coherent pluggable modules such as 400ZR.
• Coherent receivers. The local oscillator of a coherent receiver is a tunable laser set to the incoming channel, and the linewidths of the transmitter and receiver lasers together set the phase noise that the receiver's signal processing has to track.
• Swept component testing. A laser swept across a band while a detector records the transmission of a filter, an arrayed waveguide grating or a ring measures the spectrum with a resolution set by the laser's linewidth and step rather than by a spectrometer. A resonance with a loaded Q of ${10}^6$ at 1550 nm is 1.55 pm (193 MHz) wide (quality factor), so measuring it takes steps under 0.1 pm, a linewidth well below 193 MHz and a sweep without mode hops, which would appear as steps in the measured spectrum (Loaded and Intrinsic Q Factor Extraction from Ring Resonator Transmission Spectra).
• Reflectometry and ranging. OFDR resolves reflections 0.1 mm apart in fiber with a 1 THz sweep, and FMCW lidar reads range from the beat between a chirped laser and its delayed return.
• Swept-source OCT. In optical coherence tomography a 150 nm sweep at 1310 nm corresponds to an axial resolution of about 5 µm in air, taking the sweep as a Gaussian spectrum of that width.
• Spectroscopy and sensing. Current-tuned DFB lasers scan single gas absorption lines for trace-gas detection, and swept lasers read fiber Bragg grating sensors by tracking each grating's reflection peak as strain and temperature move it.

Common questions

What is a tunable laser?

A laser whose wavelength can be set anywhere within a range by a control input, such as a grating angle, a temperature or the currents through grating sections. It tunes by moving a wavelength filter inside its cavity and keeping a cavity mode aligned with it.

What types of tunable laser are there?

In telecommunications and test, semiconductor designs: temperature-tuned DFB lasers and DFB arrays, three-section and sampled-grating DBR lasers, external cavity lasers with gratings or ring filters, and MEMS-tunable VCSELs. Outside the telecom bands, Ti:sapphire and dye lasers, optical parametric oscillators, and external cavity quantum cascade lasers in the mid-infrared.

How far can a laser be tuned?

It depends on the design: about 3 nm for a DFB laser over 35 K of temperature, several nanometers for a three-section DBR laser, tens of nanometers for sampled-grating and external cavity lasers, 150 nm for a MEMS-tunable VCSEL at 1310 nm, and about 650 to 1100 nm for Ti:sapphire. Optical parametric oscillators tune over hundreds of nanometers or more.

What is mode-hop-free tuning?

Tuning in which the laser stays on one cavity mode, so that its frequency changes continuously. The optical length of the cavity has to change in proportion to the wavelength, which an external cavity laser achieves by turning its grating about the correct pivot, and a DBR laser, over part of its range, with a phase section.

Why do optical networks use tunable lasers?

One tunable laser can serve any channel of a DWDM system, so a single part replaces a set of fixed-wavelength lasers in inventory and as spares, and wavelengths can be assigned or changed while the network runs. Coherent receivers also use a tunable laser as their local oscillator.

References: L. A. Coldren et al., "Tunable semiconductor lasers: a tutorial," Journal of Lightwave Technology 22, 193 (2004). J. Buus, M.-C. Amann and D. J. Blumenthal, Tunable Laser Diodes and Related Optical Sources, 2nd ed. (Wiley-IEEE Press, 2005). F. J. Duarte (ed.), Tunable Lasers Handbook (Academic Press, 1995). L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012). V. Jayaraman, Z.-M. Chuang and L. A. Coldren, "Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings," IEEE Journal of Quantum Electronics 29, 1824 (1993). Y. Tohmori et al., "Over 100 nm wavelength tuning in superstructure grating (SSG) DBR lasers," Electronics Letters 29, 352 (1993). M.-C. Amann and R. Schimpe, "Excess linewidth broadening in wavelength-tunable laser diodes," Electronics Letters 26, 279 (1990). K. Liu and M. G. Littman, "Novel geometry for single-mode scanning of tunable lasers," Optics Letters 6, 117 (1981). V. Jayaraman et al., "High-sweep-rate 1310 nm MEMS-VCSEL with 150 nm continuous tuning range," Electronics Letters 48, 867 (2012). R. Huber, M. Wojtkowski and J. G. Fujimoto, "Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography," Optics Express 14, 3225 (2006). P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al₂O₃," Journal of the Optical Society of America B 3, 125 (1986). A. Hugi et al., "External cavity quantum cascade laser tunable from 7.6 to 11.4 µm," Applied Physics Letters 95, 061103 (2009). ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM frequency grid.