Applications of ultrafast (femtosecond and picosecond) laser systems often require the ability to tune or shift the output wavelength. Laser manufacturers have responded to this need with a wide range of interrelated products that reflect the diverse range of applications for tunable pulses-together with various options and accessories, the entire product family tree can appear quite complex. A comparison of the relative merits of three basic system architectures that deliver tunable output can simplify the process of choosing a particular system configuration to meet an application need.
In a modelocked laser system, the shortest possible pulsewidth is determined by the Fourier transform of the spectral bandwidth of the pulse; a wider spectral bandwidth supports shorter pulses. Titanium-doped sapphire (Ti:sapphire) offers the widest gain bandwidth of any currently available material, enabling systems to deliver pulsewidths shorter than 10 fs and making it the material of choice for today’s state-of-the-art ultrafast laser systems.
The broad gain bandwidth of Ti:sapphire also provides another important advantage for many users: the ability to tune the center wavelength of the output pulses over this bandwidth (see Fig. 1). The simplest tunable ultrafast system is thus a Ti:sapphire oscillator pumped by a 532 nm solid-state continuous-wave (CW) laser. Moreover, the same basic Ti:sapphire laser can be configured for either femtosecond or picosecond operation by constraining the spectral bandwidth.
In dispersive materials such as lenses, Ti:sapphire crystals, and even dielectric coatings, the different wavelength components of an ultrafast pulse travel at slightly different velocities. The effect of this group velocity dispersion (GVD) is to stretch the overall temporal profile of the laser pulse, becoming a critical concern in the femtosecond range. These effects of GVD must be compensated to achieve the shortest possible (transform limited) output pulse. A simple way to do this is to incorporate a matched pair of dispersive prisms within the Ti:sapphire laser cavity. The prisms serve to make the effective cavity length a function of wavelength and can be used to create a GVD effect that is identical in magnitude, but opposite in sign, relative to the cumulative effect of the other optical elements in the laser cavity.
The Ti:sapphire laser is tuned by rotating a birefringent filter that acts as a bandpass filter for the polarized intracavity beam. If the center wavelength is changed by a few tens of nanometers or less, that is the only adjustment required. For large changes in wavelength, one of the prisms in the prism pair must be adjusted because cavity GVD changes with wavelength.
The pulse repetition rate is very high for a modelocked Ti:sapphire oscillator, with a typical value of 76 MHz. Depending on the pump power used, the output power of a Ti:sapphire oscillator can exceed 4 W, which translates into pulse energies at the 50 nJ level. But because of the short pulse duration, the peak power can be as high as 0.5 MW. This enables harmonic conversion and other nonlinear wavelength-shifting techniques to be used to extend wavelength coverage-important because the typical Ti:sapphire oscillator has a spectral gain bandwidth covering only 680 to nm. Utilizing the second harmonic accesses the 350 to 540 nm wavelength range and the third harmonic delivers 225 to 330 nm. Furthermore, the pulse energies of second-harmonic and third-harmonic generation can be significantly enhanced by cavity dumping the Ti:sapphire oscillator. This “bolt on” accessory for the Ti:sapphire oscillator boosts the oscillator pulse energy by up to 10 times, albeit with a corresponding reduction in pulse repetition rate.
Ti:sapphire oscillators support a range of applications depending upon system wavelength and power. The low pulse energy is well suited to condensed-phase experiments in physics, chemistry, biology, and electrical engineering. The high sample density means that the signal can exceed the shot noise even with nanojoule pulses, and the extremely high repetition rate allows statistical averaging over many millions of pulses to be achieved in only seconds. Conversely, “tunable” applications that can need more than just a Ti:sapphire oscillator include gas-phase experiments in which the signal would be less than the laser shot noise, and/or experiments requiring wavelengths in the infrared or in the so-called “Ti:sapphire gap” between 500 and 700 nm.
The simplest way to access the Ti:sapphire gap and/or wavelengths longer than 1 µm is to use a synchronously pumped optical parametric oscillator (OPO) in conjunction with the Ti:sapphire oscillator. An OPO relies on a nonlinear process called parametric down-conversion, which can be considered to be the opposite of sum-frequency mixing, and thus requires a nonlinear crystal. In an OPO, an input “pump” photon is split to produce two photons of lower energy, referred to as the signal and idler photons, in which the sum of the two photons conserves the original photon energy. By convention, the high-photon-energy (shorter-wavelength) output is referred to as the signal beam; the idler is the lower-energy photon beam. The OPO process also conserves photon momentum, which requires a phase-matching condition and determines the wavelength of the signal (and hence idler). The phase-matching condition can be adjusted by changing the pump-laser wavelength, the angle of the nonlinear crystal, or the crystal temperature-the three methods by which the output(s) of an OPO is tuned.
Like a Ti:sapphire laser, the OPO can be operated in the femtosecond or picosecond domain. But unlike a laser, the OPO crystal does not store any optical gain-it will only emit light during the time it is pumped. For this reason, the cavity of an ultrafast OPO is set to the same length as the Ti:sapphire cavity, or for the same 76 MHz repetition rate. Every time the signal pulse circulating in the OPO passes through the nonlinear crystal, this crystal is simultaneously being pumped by a pulse from the Ti:sapphire oscillator, delivering gain to the signal and idler beams within the OPO cavity (hence the name “synchronously pumped OPO”). During each pass around the OPO, part of the signal beam leaves the cavity as an output beam through a partially reflecting cavity optic.
There are many variants on the basic OPO format, two of which merit further examination. The first is the use of an optional ring-cavity configuration. This approach generates a high-power intracavity signal beam. An intracavity doubler is then used to convert this near-infrared beam to a visible wavelength, delivering high power over the entire Ti:sapphire gap.
The second variation utilizes periodically poled OPO crystals, which have two advantages compared to bulk crystals such as potassium titanyl phosphate (KTP). Most important, the OPO can be automatically tuned by adjusting the OPO output coupler to lengthen (or shorten) the cavity (see Fig. 2). In femtosecond operation, for example, the pump wavelength is fixed at approximately 830 nm for OPO tuning over a 500 nm infrared range. In contrast, bulk-crystal OPOs require tuning of the Ti:sapphire oscillator wavelength. In addition, the periodically poled material provides direct access to wavelengths as long as 3.3 µm. Even longer wavelengths can be generated by difference-mixing of the signal and idler beams.The world is moving towards tunability. Datacom and telecom companies may increase their network capacity without investing in new fiber infrastructure thanks to tunable lasers and dense wavelength division multiplexing (DWDM). Furthermore, the miniaturization of coherent technology into pluggable transceiver modules has enabled the widespread implementation of IP over DWDM solutions. Self-tuning algorithms have also contributed to the broad adoption of DWDM systems since they reduce the complexity of deployment and maintenance.
The tunable laser is a core component of all these tunable communication systems, both direct detection and coherent. The fundamental components of a laser are the following:
As light circulates throughout the resonator, it passes multiple times through the pumped gain medium, amplifying itself and building up power to become the highly concentrated and coherent beam of light we know as a laser.
There are multiple ways to tune lasers, but let’s discuss three common tuning methods. These methods can and are often used together.
With this short intro on how lasers work and can be tuned, let’s dive into some of the different tunable lasers used in communication systems.
Distributed Feedback (DFB) lasers are unique because they directly etch a grating onto the gain medium. This grating acts as a periodic mirror, forming the optical resonator needed to recirculate light and create a laser beam. These lasers are tunable by tuning the temperature of the gain medium and by filtering with the embedded grating.
Compared to their predecessors, DFB lasers could produce very pure, high-quality laser light with lower complexity in design and manufacturing that could be easily integrated into optical fiber systems. These characteristics benefited the telecommunications sector, which needed lasers with high purity and low noise that could be produced at scale. After all, the more pure (i.e., lower linewidth) a laser is, the more information it can encode. Thus, DFB lasers became the industry’s solution for many years.
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The drawback of DFB lasers is that embedding the grating element in the gain medium makes them more sensitive and unstable. This sensitivity narrows their tuning range and makes them less reliable as they age.
A simple way to improve the reliability compared to a DFB laser is to etch the grating element outside the gain medium instead of inside. This grating element (which in this case is called a Bragg reflector) acts as a mirror that creates the optical resonator and amplifies the light inside. This setup is called a distributed Bragg reflector (DBR) laser.
While, in principle, a DBR laser does not have a wider tuning range than a DFB laser, its tuning behavior is more reliable over time. Since the grating is outside the gain medium, the DBR laser is less sensitive to environmental fluctuations and more reliable as it ages. However, as coherent and DWDM systems became increasingly important, the industry needed a greater tuning range that DFB and DBR lasers alone could not provide.
Interestingly enough, one of the most straightforward ways to improve the quality and tunability of a semiconductor laser is to use it inside a second, somewhat larger resonator. This setup is called an external cavity laser (ECL) since this new resonator or cavity will use additional optical elements external to the original laser.
The main modification to the original semiconductor laser is that instead of having a partially reflective mirror as an output coupler, the coupler will use an anti-reflection coating to become transparent. This helps the original laser resonator capture more light from the external cavity.
The new external resonator provides more degrees of freedom for tuning the laser. If the resonator uses a mirror, then the laser can be tuned by moving the mirror a bit and changing the length of the resonator. If the resonator uses a grating, it has an additional element to tune the laser by filtering.
ECLs have become the state-of-the-art solution in the telecom industry: they use a DFB or DBR laser as the “base laser” and external gratings as their filtering element for additional tuning. These lasers can provide a high-quality laser beam with low noise, narrow linewidth, and a wide tuning range. However, they came with a cost: manufacturing complexity.
ECLs initially required free-space bulk optical elements, such as lenses and mirrors, for the external cavity. One of the hardest things to do in photonics is coupling between free-space optics and a chip. This alignment of the free-space external cavity with the original laser chip is extremely sensitive to environmental disturbances. Therefore, their coupling is often inefficient and complicates manufacturing and assembly processes, making them much harder to scale in volume.
Laser developers have tried to overcome this obstacle by manufacturing the external cavity on a separate chip coupled to the original laser chip. Coupling these two chips together is still a complex problem for manufacturing but more feasible and scalable than coupling from chip to free space optics. This is the direction many major tunable laser developers will take in their future products.
As we explained in the introductory section, linear resonators are those in which light bounces back and forth between two mirrors. However, ring resonators take a different approach to feedback: the light loops multiple times inside a ring that contains the active medium. The ring is coupled to the rest of the optical circuit via a waveguide.
The power of the ring resonator lies in its compactness, flexibility, and integrability. While a single ring resonator is not that impressive or tunable, using multiple rings and other optical elements allows them to achieve performance and tunability on par with the state-of-the-art tunable lasers that use linear resonators.
Most importantly, these widely tunable ring lasers can be entirely constructed on a single chip of Indium Phosphide (InP) material. As shown in this paper from the Eindhoven University of Technology, these lasers can even be built with the same basic building blocks and processes used to make other elements in the InP photonic integrated circuit (PIC).
This high integration of ring lasers has many positive effects. It can avoid inefficient couplings and make the laser more energy efficient. Furthermore, it enables the development of a monolithically integrated laser module where every element is included on the same chip. This includes integrating the wavelength locker component on the same chip, an element most state-of-the-art lasers attach separately.
As we have argued in previous articles, the more elements can be integrated into a single chip, the more scalable the manufacturing process can become.
Factors such as output power, noise, linewidth, tuning range, and manufacturability are vital when deciding which kind of laser to use. A DFB or DBR laser should do the job if wide tunability is not required. Greater tuning range will require an external cavity laser, but if the device must be manufactured at a large volume, an external cavity made on a chip instead of free-space optics will scale more easily. The latter is the tunable laser solution the telecom industry is gravitating towards.
That being said, ring lasers are a promising alternative because they can enable a widely tunable and monolithically integrated laser with all elements, including wavelength locker, on the same chip. This setup is ideal for scaling into high production volumes.