Mode-locked lasers are common tools for producing short pulses in the time domain, including telecommunications applications at multigigahertz repetition frequencies that require tunability in the C-band. Now they also can work as multiwavelength sources in wavelength division multiplexing (WDM) applications. Both cost-effectiveness and performance are fundamental requirements of today’s WDM systems, which are built using multiple wavelengths at precise locations on the ITU grid. Because mode-locked lasers produce a comb of high-quality channels separated precisely by the pulse repetition frequency, one source can replace many of the distributed feedback lasers currently used. Channel spacing can range from greater than 100 GHz to 3.125 GHz. This single-source solution for WDM system architectures can reduce costs and enable applications in metro and access networks, test and measurement instrumentation, and portable field test equipment. New applications, such as supercontinuum generation, frequency metrology and hyperfine distributed WDM, also could benefit from the lasers’ spectral and temporal properties. Mode-locking The output of mode-locked lasers in the time domain is a continuous train of quality pulses, which in this example exhibits a 25-GHz repetition rate, a 40-ps period and a pulse width of approximately 4 ps (Figure 1). In general, a laser supports modes at frequencies separated by a free spectral range of c/2L, where L is the cavity length. Often a laser has multiple modes, with mode phases varying randomly with time. This causes the intensity of the laser to fluctuate randomly and can lead to intermode interference and mode competition that reduces its stability and coherence. Stable and coherent CW lasers usually have only one mode that lases. Figure 1. Each 25-GHz optical pulse from a mode-locked erbium-glass laser at 1535 nm has a width of 4 ps FWHM, 40-ps repetition period, extinction ratio more than 30 dB and average power exceeding 10 dBm (left). In the wavelength domain spectrum of the same laser measured with an optical spectrum analyzer with 0.01-nm resolution (right), the noise floor is flat between the channels. The rise in the center is caused by the analyzer’s internal filter. Mode-locking produces stable and coherent pulsed lasers by forcing the phases of the modes to maintain constant values relative to one another. These modes then combine coherently. Fundamental mode-locking results in a periodic train of optical pulses with a period that is the inverse of the free spectral range. The pulsation period is the interval between two successive arrivals of the pulse at the cavity’s end mirrors. There is a fixed relationship between the frequency spacing of the modes and the pulse repetition frequency. In other words, the Fourier transform of a comb of pulses in time is a comb of frequencies or wavelengths. This capability is key to making a mode-locked laser a multi-wavelength source. Mode-locking occurs when laser losses are modulated at a frequency equal to the intermode frequency spacing. One way to explain this is to imagine a shutter in the laser cavity that opens only periodically for short intervals. The laser can operate only when the pulse coincides exactly with the time the shutter is open. A pulse that operates in this cavity would require that its modes be phase-locked, and the shutter would trim off any intensity tails that grow on the pulses as the mode phases try to wander from their ideal mode-locked values. Thus, a fast shutter in the cavity has the effect of continuously restoring the mode-locked condition. Mode-locked lasers operate at repetition frequencies and pulse widths that require much higher performance than a mechanical shutter can offer. There are two basic ways to modulate the losses in the laser cavity to achieve mode-locking. Actively mode-locked lasers usually employ an electro-optic modulator driven by a radio-frequency signal at the repetition frequency of the cavity. Passively mode-locked lasers, on the other hand, employ devices called saturable absorbers to spontaneously lock the modes with fast material response times, without the use of an external drive signal. Fiber, semiconductor and erbium-glass lasers are among the mode-locked devices used at telecommunications wavelengths. Fiber lasers are usually actively mode-locked at a harmonic of the final repetition frequency. Their cavities are long because a long fiber is required to obtain sufficient gain. They tend to be relatively large and complex, but offer flexibility in parameter adjustment and high output powers. Semiconductor lasers are also actively mode-locked, in most cases. These small devices, which tend to have relatively low power and stability, are still a developing technology in research laboratories. The passively mode-locked erbium-glass laser, on the other hand, is a simple high-performance platform (Figure 2). The cavity comprises the gain glass, laser mirrors, a saturable absorber and a tunable filter. The cavity is short for 25-GHz lasers at approximately 6 mm, allowing a compact device that also offers high output power (see inset). In this context, passive mode-locking means that the CW pump laser is focused into the cavity at 980 nm and that picosecond pulses emit from the cavity at 1550 nm, with no other inputs or signals required. Figure 2. This erbium-glass multiwavelength laser focuses a 980-nm CW pump into the erbium gain glass. A saturable absorber provides passive mode-locking, so no active signal is required. The cavity length for the 25-GHz laser is 6 mm. The erbium-glass device takes advantage of the maturity of components used in erbium-doped fiber amplifier products, and it is optically pumped with an industry-standard 980-nm diode. These pumps are becoming cheaper and more robust even as they achieve higher output powers and stability. The current average output power of the multiwavelength laser across the C-band is 10 dBm. This device has a saturable absorber combined with a reflective substrate to create a semiconductor saturable absorbing mirror with reflectivity that increases with optical intensity. It is an ultrafast optical switch that acts like an intracavity shutter to produce the mode-locked spectrum. This has the effect of accumulating all the lasing photons inside the cavity in a very short time with a very high optical fluence. The mirror also has response time on the order of femtoseconds for pulse formation and picoseconds when it is time to initiate self-start of the laser. The proprietary component is made with fundamental semiconductor techniques. The erbium-glass laser is tunable through the C-band so that the comb of wavelengths can be set to cover any section of grid channels from 1530 to 1565 nm. Locking to the ITU grid requires the multiwavelength comb to be shifted in frequency to coincide exactly with the known reference grid, where it is then locked. The maximum frequency shift needed would be the comb spacing, which is equal to the free spectral range of the mode-locked laser. A shift of one free spectral range in the laser requires a cavity length change of one wavelength, which is 1.5 μm. Filtering out one channel of the comb’s edge then allows ITU grid locking with minor cavity adjustment. WDM channel generation By combining the erbium-glass multiwavelength laser with other available telecommunications components, it is possible to make a multichannel WDM source (Figures 3a and b). The laser is connected to a dynamic gain equalizer and an erbium-doped fiber amplifier to produce a flattened 32-channel distributed WDM wavelength comb with channel linewidth on the order of 1 MHz. Figure 3a. In this multiwavelength platform setup, a dynamic gain equalizer flattens and filters the laser’s spectrum. An erbium-doped fiber amplifier increases channel power. Using one channel, one wavelength locker and a cavity adjustment of less than 1 μm, the entire wavelength spectrum can be locked to the ITU grid. In this application, engineers set the 25-GHz comb-generating laser to a center wavelength of 1535 nm and an average power of 12 dBm. With this device, the optical signal-to-noise ratio for the modes in the center of the output spectrum is typically greater than 60 dB. Numerous locked modes extend in each direction from the center of the spectrum, with decreasing power and signal to noise. Thus, the number of usable channels from the multiwavelength laser can be defined using comparable signal-to-noise requirements of current WDM sources. Figure 3b. The multiwavelength laser platform produced this 32-channel WDM channel grid. Signal to noise is greater than 30 dB, and the channels are separated by exactly 25 GHz on the ITU grid. Channel flatness is less than 0.4 dB. Because the laser is fundamentally mode-locked, there are no side modes between the channels, but the side-mode-suppression ratio of a typical distributed feedback laser can be used as a threshold for the signal-to-noise requirements of the channels from the multiwavelength laser. Typical suppression ratios for WDM laser sources are around 35 dB. More than 32 modes have ratios greater than 35 dB in the multiwavelength spectrum, so this test can be run using 32 channels. Comb flattening The dynamic gain equalizer allows flattening the comb of 32 channels and attenuating the modes outside the desired comb bandwidth. The erbium-doped fiber amplifier takes the channels to power levels consistent with WDM applications. In one test, channel powers were demonstrated up to levels of 10 dBm. It also is possible to set the profile of the equalizer to account for the amplifier’s gain profile. This allows optimization of the system for channel count, signal to noise and power. The optical spectrum analyzer used to capture the DWDM spectrum has 0.01-nm resolution. The gain equalizer in this example is a product from Silicon Light Machines of Sunnyvale, Calif., that has high enough resolution to support any channel spacing throughout the C-band. The device acts as an addressable diffraction grating with numerous narrow ribbons of individual microelectromechanical systems in a long row. The relative power accuracy and spectral power ripple are ±1 dB. The dynamic range is greater than 15 dB. The test setup has a standard erbium-doped fiber amplifier with a saturated output power of 27 dBm. Besides providing a platform to test WDM components, the mode-locked source can be used to demonstrate production of a supercontinuum spectrum. Scientists have used highly nonlinear fibers with decreasing dispersion profiles to extend multiwavelength combs to cover up to 300 nm of optical bandwidth. The high peak power of the picosecond pulses interacts with the nonlinear fiber to produce the supercontinuum. Pulses from the 25-GHz erbium-glass laser are a good fit with the requirement of supercontinuum generation. Myriad applications This capability can open up many new applications by generating more than 1000 high-quality optical carriers for distributed WDM, enabling multiwavelength short pulses for optical time division multiplexing and WDM and producing precision optical frequency grids for frequency metrology. Another advanced application is hyperfine distributed WDM, which transmits slower data rates on very densely spaced channels as close as 3.125 GHz. The slower data rates simplify the electronics, avoid added time division multiplexing and eliminate the serious dispersion problems suffered by higher-speed signals, particularly at 40 GHz. Multiwavelength lasers are uniquely suited to this application because of their ability to generate many channels with a single source at very high densities. In essence, a variety of practical solutions to current and near-term challenges are possible with the multiwavelength platform. WDM systems must compete in an increasingly demanding environment in terms of cost, size, power consumption and complexity. A multiwavelength platform allows new and more efficient architectures to be developed and tailored for specific applications. Meet the author Michael Brownell is vice president of product development at GigaTera in Dietikon, Switzerland.