Use of fast wavelength tunable laser modules Essay Example

wavelength division multiplexing (WDM) system with multiple TLs.

Another focus of the research is the characterization and system impact of TL wavelength drift. The initial wavelength drift of a tuned TL module has been determined, and two approaches (extended blanking and sub-carrier multiplexed transmission) have been demonstrated to overcome this drift. This allows for error-free transmission in an ultra-dense WDM system with a spacing of 12.5 GHz.The Optically Labelled Packet Switched System was able to correlate the spot error rate with the wavelength impulse from a TL facility. In this system, the light source used for DWDM channels is of utmost importance and is often the most expensive component in the network. The preferred light emitters are high-resolution precise narrowband semiconductor optical lasers made with Indium Gallium Arsenide Phosphide (InGaAsP) on an Indium Phosphide substrate.

In today's networks, light is launched onto any of the supported wavelengths using specifically fabricated single wavelength distributed feedback optical lasers (DFBs). These DFBs operate on fixed wavelengths as specified in the ITU recommendation G.694.1 for DWDM networks with various channel separations.

Important laser parameters for network operation include output power, side mode suppression ratio (SMSR), wavelength stability, line width, and lifetime. A description of current and future DWDM tunable semiconductor optical lasers can be found in Chapter 2.

Tunable lasers (TL) development will play a crucial role in the success of the potential optical networks discussed in the previous sections.TLs are currently being used in optical webs for saving, dynamic provisioning, and channel restoration. These applications require tuning velocities in the millisecond range. However, the introduction of OBS will increase the need for even faster tuning speeds, on the order of microseconds. In

the future, TLs may also be used for optical shift of single packages (OPS), where TLs would serve as tunable senders or local optical masers in tunable wavelength converters. The switching speeds in these types of webs will be limited by the tuning velocity of the TL, and tuning times of nanoseconds or less will be necessary to maintain efficiency. Demand for TLs is expected to grow rapidly, with a compound annual growth rate of 37% over the next five years. This growth is driven by WDM in long haul and metro webs transitioning to agile webs using ROADMs. In Chapter 2, we will examine the technology that enables TLs to have tunable wavelength functionality, building upon the discussion of current and future DWDM systems in Chapter 1.The text presents the TL applications and chief demands in such systems. It introduces the basic tuning mechanisms and strategies for a generic tunable optical laser and discusses the execution of this tuning function in individual wavelength electronic TLs. The discussion is further carried forward to explore widely tunable electronic Thallium with a particular emphasis on the sampled grating distributed Bragg reflector optical laser.

2.1 Applications of Tunable Lasers

Research, development, and deployment of TL engineering have been ongoing for some time [1]. The application of TLs in increased capacity optical telecommunications systems has driven much of the interest. Single frequency laser rectifying tubes with tunable wavelength functionality are important components in current and future wavelength multiplexed optical communication systems. The use of TLs in the areas of consistent optical communication, sensing, and measurement is also of interest. However, these applications are outside the scope of this

thesis, which focuses on the use of TLs in WDM systems.

2.1.1 Sparing and Inventory Reduction

The most obvious application of TLs in current DWDM systems, where increasing numbers of wavelength channels are being used, is as optical sources to replace single fixed frequency DFB lasers. This will reduce costs and improve the simplicity of manufacturing and operating DWDM systems.By utilizing TLs (Tunable Lasers), stock list can be reduced as each individual TL can operate at any required wavelength. This means there is no need to have a separate laser for each wavelength channel in case of a laser weakness, resulting in cost savings. TL technology also offers interesting applications in optical shift, routing, and networking. TLs will be integrated into future optical routing and networking architectures. In these architectures, data will be encoded onto wavelength channels specific to each destination. To route the data, passive wavelength selective filtering like NA-N AWG can be used. The ultimate goal is to perform routing for each data package, and TLs with nanosecond or faster tuning times will be needed. Reconfigurable Optical Add-Drop Multiplexers (ROADMs) are tunable OADMs capable of changing the wavelengths dropped and added to a set of DWDM signals on a fiber. This allows for dynamic provisioning of wavelength channels, making networks more manageable and scalable.The primary function of channel multiplexing and demultiplexing is performed by tunable filters. However, when the added wavelength is not known in advance, tunable lasers (TLs) will be required to allow transmission on the desired wavelength.

Wavelength converters, specifically tunable wavelength converters (WCs), are crucial components in both current and future DWDM networks. The ability to convert a high data

rate signal from any input wavelength channel to a tunable output wavelength channel is important for increasing network flexibility and enabling data to be routed along different paths. There are various approaches to performing conversion, ranging from optical-electronic-optical (OEO) WCs that detect the signal before retransmission on the new wavelength, to all-optical WCs that use the optical nonlinearities of certain materials to optically modulate the new wavelength.

Regardless of the approach used, a local laser is necessary to set the new wavelength to which the signal is converted. To achieve tunable wavelength conversion, a TL is required to act as the local laser. Wavelength converters play a vital role in optical switching. In wavelength-division multiplexing (WDM), most of the switching for connecting networks is done through wavelength exchange. Tunable wavelength converters can be combined with non-blocking arrayed waveguide gratings (NA-N AWGs) to form non-blocking optical wavelength switches.The requirements for tunable optical lasers in DWDM systems may vary depending on the specific application. However, they should generally have the same performance as fixed wavelength lasers currently used in DWDM systems, such as distributed feedback lasers, but with the additional functionality of wavelength tuning. Table 2.1 outlines the key requirements for tunable lasers [6, 7].

The process of wavelength tuning in a generic laser diode is described in this section, referencing [8] and [9]. By adjusting the cavity length and/or longitudinal mode positions, the laser emission wavelength can be tuned based on the underlying laser spectra.

In a simplified laser structure (Figure 2.1), when current is applied to the active region, carriers are initially converted to photons through spontaneous emission. The emitted photons are reflected back into the active region

for amplification through mirror reflection at the cavity ends. This amplification produces coherent photons of the same wavelength and phase. The lasing process occurs when the roundtrip gain of the cavity reaches unity, creating the steady state condition required for laser operation.This occurs when the wavelength-dependent addition, given by the optical gain ( I“ ) of the active medium (tabun), exceeds the internal losses (I±i) and mirror losses (I±m), resulting in the gain condition. When this gain threshold is surpassed, any additional injected carriers are directly converted to photons through stimulated emission. This establishes the amplitude condition that defines the laser's lasing state, the roundtrip gain characteristic of the cavity, as shown in Figure 2.2 (a). The phase condition of the cavity relies on the cavity length (L) and effective refractive index (n) for the Nth mode centered at wavelength i?¬N. This defines a set of longitudinal cavity modes, depicted in Figure 2.2 (B), with mode spacing i?¬m, where lasing can occur if the gain condition is met. The laser's wavelength is determined by the longitudinal mode closest to the gain peak, I»p, of the cavity gain characteristic. In the illustrated case in Figure 2.2 (c), which is typical of a Fabry-Perot (FP) Laser, the laser emission wavelength is centered at I»N with side modes carrying a significant portion of the total laser power. Achieving single-mode operation can be done by making the mirror loss wavelength-dependent, as described in section 2.3.The wavelength of the optical laser can be adjusted by changing the amplitude status, which switches the peak intensity of the pit addition characteristic, or by changing the stage status, which switches the longitudinal comb

manners. Alternatively, a combination of both methods can be used. The specific tuning method chosen will determine the type of tuning achieved: continuous, discontinuous, or quasi-continuous. It is important to maintain other laser parameters, such as side mode suppression ratio (SMSR) and output power, as constant as possible during wavelength tuning.

In continuous tuning, the laser wavelength is smoothly adjusted in small increments while remaining in the same longitudinal mode throughout the tuning range. This requires simultaneous control of the peak wavelength of the pit addition and the spectrum of the comb mode. By maintaining other laser parameters constant and staying in the same dominant longitudinal mode, the tuning range is limited to around 15 nanometers. The spectrums involved in continuous tuning include: cavity gain characteristic spectrum, longitudinal mode spectrum, and longitudinal laser emission spectrum.

In discontinuous tuning, if the laser is not restricted to the same operational mode, the wavelength can be adjusted across the longitudinal modes of the laser.This method of manner hopping allows for a wider range of tuning, up to approximately 100 nanometers, which is determined by the tuning capabilities of the pit addition characteristic. However, it is not possible to access every wavelength within this range.

The strategy of quasi-continuous tuning involves combining multiple overlapping continuously tunable ranges. By using manner hopping and continuous tuning within each manner, a large tuning range can be achieved where every wavelength is accessible.

The tuning of the cavity gain characteristic can change the output wavelength of the laser. This is done by adjusting the spectral dependence of the pit addition curve. This adjustment can be achieved by changing the wavelength dependence of the active medium gain or

by using the mirror loss as a wavelength selective filtering component. Assuming no changes in the comb mode spectrum, the laser wavelength will change in mode hops as the pit wavelength is varied. This results in discontinuous tuning, as shown in Fig.2.3 (a). However, a review of the output emission spectrum reveals that the issue is more ambiguous as the wavelength is tuned across different modes.(a) Laser wavelength as a map of I»p displacement: The SMSR varies during tuning due to the fluctuation in the addition difference between the dominant manner and the 2nd strongest manner. This wavelength ambiguity is illustrated utilizing the markers x, y, and omega in Fig.2.3 for the manner passage from I»N to I»N-1. The best suppression is given when I»p coincides with one of the comb manners, ab initio I»N, place ten. The addition difference between the two viing manners lessenings, until the eventual laterality of the following manner; it so begins to increase until I»p coincides with mode I»N-1, place omega. The lowest suppression value is given at the manner hop boundary, place Y. This tuning method is finally limited by the tuning scope over which I»p can be tuned.

(b) Laser emanation spectra at indicated I»p places: The optical maser wavelength can besides be tuned by switching the comb manner spectrum. With mention to equation (2.3), this can be achieved by either altering the length (L), or by altering the effectual refractile index (n), of the optical maser pit. Within the wavelength scope of involvement all the comb manners can be regarded as holding an equal spectral displacement. Assuming no alteration in the pit addition peak the wavelength will

alter as the comb manner spectrum, I»c, is varied as shown in Fig.For the purpose of illustration, positional markers x, y, and omega are once again used. At position x, the comb is placed in such a way that manner N aligns with I»p. As the comb is shifted, the wavelength continuously and linearly changes until position Y. Then, the laser jumps to the neighboring manner, N+1, resulting in a downward wavelength shift equal to the spacing of the manners. This shifting continues, and the wavelength increases until it reaches omega, where manner N+1 aligns with I»p. This tuning process creates periodic and continuous wavelength regimes with a width of I”I»m, centered at the initial starting wavelength I»N. However, after each period, there is a leap to the next manner, resulting in mode ambiguity. This cancellation of previous tuning efforts limits acceptable continuous tuning to a small range of wavelengths within I”I»m. The same problems with SMSR associated with tuning of the pit addition curve can be seen in Figure 2.4 (B). Therefore, achieving satisfactory continuous tuning requires simultaneous control of both the pit addition peak and the comb mode spectrum.By tuning the comb mode spectrum by the same amount as the pit addition maximum (i.e. p = c), it is possible to achieve a relatively wider tuning range. This approach, shown in Fig.2.5 (a), provides continuous tuning with fixed SMSR throughout the entire range. The tuning range is determined by the smaller range of p or c, typically limited by c, which is slightly larger than the continuous tuning range described in section 2.2.2, which is a fraction of m. This range can be

further increased by using a quasi-continuous tuning method. Similar to before, p is monotonically increased while c is changed stepwise over m, resetting to its initial value and causing a mode jump to a lower mode. The resulting wavelength increases smoothly as in the continuous strategy with fixed SMSR, but mode jumps are allowed after each longitudinal mode. However, this periodic mode skipping introduces uncertainty in terms of phase and wavelength around the mode boundaries, as shown in Fig.2.5 (B). The emission spectrum remains the same before and immediately after the mode hop, despite the dominant mode changing from N to N-1. This hinders certain applications such as consistent optical sensing, but it provides the widest tuning range and is therefore attractive for use in WDM applications.Laser emission spectra utilizing combined tuning for (a) uninterrupted tuning and (B) quasi-continuous tuning are necessary for single mode operation. In an FP optical maser, multiple modes reach the lasing state, resulting in a multimode emission spectrum due to broad spectral gain and closely spaced longitudinal modes. However, for DWDM systems, single mode operation is required to address issues like chromatic scattering and SMSR. One method to achieve single mode operation is by using shorter pits to increase the mode spacing so that only one mode falls under the gain curve, as seen in VCSEL. Another widely employed method is the use of periodic corrugated structures to provide mode selectivity. In an FP optical maser, mirror losses are wavelength independent, and the spectral emission consists of modes that experience net gain determined by the broad gain curve. Mode selection filtering can be improved by using periodic structures to create

wavelength dependent mirror loss, suppressing all modes except the selected lasing mode, as shown in Fig. 2.6.In a wave guide with an intermittently changing index grating, the reflection coefficients for different modes will vary depending on the wavelength. Distributed feedback occurs when the wavelength aligns with the grating's reflections, known as the Bragg wavelength. The Bragg wavelength is determined by the grating pitch (?) and the effective refractive index (n). The mode closest to the Bragg wavelength (?B) will be reflected positively since the reflections are in phase. Modes deviating from ?B will have out-of-phase reflections and will be suppressed. This results in only a single mode reflection at ?B, allowing for lasing specifically at this mode. Laser emission can be achieved using mode selective filtering centered at the Bragg wavelength (?B).

2.4 Electronic Tunable Lasers

It has been demonstrated that the lasing wavelength of a laser can be adjusted by altering the phase and/or amplitude conditions. Equation 2.3 represents the phase condition, showing that the positions of the comb modes can be shifted spectrally by modifying either the physical length (L) or the effective refractive index (n) of the laser cavity.In order to achieve swift adjustability, it is impractical to physically set the length L for mechanical or electromechanical tuning. This limitation restricts the tuning time to milliseconds. A possible solution is to use a waveguide section with an electronically controllable refractive index as a suitable tuning component.
Alternatively, for amplitude control, a Bragg grating reflector can be employed to produce wavelength selective loss, enabling lasing only at the wavelength of minimum loss - the Bragg wavelength (?B). According to equation 2.4, the pitch

of the reflector (?) cannot be easily varied dynamically as it is fixed in fabrication. However, the ?B can be tuned by electronically controlling the effective refractive index of the grating. This approach has the potential to provide a tuning time for optical shift applications that is short enough.
To tune the output wavelength electronically, it is necessary to alter the refractive index of either a phase component or an amplitude component. Refractive index adjustment can be accomplished in semiconducting materials through various methods such as field effects, thermal control, and carrier injection.

2.4.1 Refractive Index Tuning

The use of an electric field allows for changing the refractive index of a material, as demonstrated in Mach-Zehnder interferometer (introduced in section 1.3.2).This electro-optic consequence has high velocity tuning capability, but it only provides a small index displacement [ 11 ] . This limitation restricts its potential tuning range, making it unsuitable for DWDM TL applications. Thermal control can also be used for wavelength control by increasing the refractive index with temperature [ 12 ] . However, excessive warming will ultimately limit the achievable tuning range. The tuning velocities are also limited to microsecond time scales due to thermal resistance. The most commonly used method for controlling the refractive index of a semiconductor wave guide for wavelength tuning is carrier injection, which yields the largest index displacement at nanosecond timescales [ 9 ] . By injecting carriers into the wave guide through an external current source, the effective index is reduced proportionally to the additional carrier density, resulting in an approximate effective index change ( I”n ) described by the equation where _ is the optical mode

confinement factor, _ is the index change per carrier density, and N is the injected carrier density [ 13 ] . In the case of a Bragg reflector, carrier injection decreases the effective index, causing a negative shift in the Bragg wavelength ( I»B ) according to equation 2.4. To maintain I»B at this wavelength, the current source must be sustained at an appropriate level or changed for a different wavelength shift.The tuning range of the I»B is limited by the sum of effective index alteration achievable in the grate. However, this index alteration becomes less efficient at higher injection current levels due to increased nonradiative recombination at higher carrier densities. Additionally, excessive warming of the laser from sustained current injection will also restrict the tuning range. This warming not only affects laser parameters such as power and threshold current, but also causes a parasitic refractive index increase that partly counters the carrier injection index alteration.

Regarding the Distributed Feedback (DFB) Laser, it combines an active and grating section along the length of the laser cavity. Originally developed as a fixed wavelength single mode laser, it has become one of the most prevalent lasers. The integration of wavelength selectivity and gain functionality allows for relatively simple fabrication without an active/passive interface. Only wavelengths near the Bragg wavelength are reflected back into the cavity, creating a limited range of light that builds up within the active layer and reaches the lasing threshold (as depicted in Fig. 2.7).The schematic of a Distributed Feedback (DFB) Laser shows that early tunable lasers (TLs) were based on DFB technology, with the electrode split into two or three tuning subdivisions. However, the

operation of these devices is complex and wavelength control is difficult, offering only modest tuning scopes of around 3 nm. In comparison, the use of thermally controlled DFBs as TLs is more common and provides a stable mode-hop-free wavelength tuning that is easily controlled using standard DFB engineering principles. Since the emission wavelength of a DFB changes by approximately 0.1 nm/oC, it is possible to achieve a tuning scope of 3-4 nanometers by temperature tuning within the range of 30-40 oC.

To achieve wider tuning scopes, various component sellers have developed selectable arrays consisting of 8-12 DFB lasers that can be operated at any wavelength across the C-band or large portions of it. The operating wavelength of the array can be determined either through optical coupling or by using an external micro-electromechanical (MEM) mirror. By utilizing DFBs with different grating periods to produce default output wavelengths that are spaced approximately 3 nm apart, it becomes possible to cover the C-band using a thermally controlled 12 DFB array. However, there are drawbacks associated with using arrays, such as power loss due to coupling or travel-related losses associated with the MEM mirror option. Additionally, the slow tuning times reported, which are measured in seconds, limit their use for future dynamic functionality.The

Distributed Bragg Reflector (DBR) Laser

is a type of laser that was initially developed for fixed wavelength operation, similar to DFBs. However, DBRs are more suitable for wavelength tunability compared to DFBs. This is because DBRs have a built-in separation between the active and inactive parts, reducing the impact of wavelength tuning on the laser's functionality. The most important DBR laser, shown in Fig.2.8, is

a three-section device with a separate inactive wave guide section. At one end of the device, there is a Bragg grating which acts as a wavelength selective mirror. An anti-reflection (AR) coating is applied to this end to reduce reflections. The other end of the device provides mirroring through a cleaved aspect. A separate inactive phase control section separates the grating section from the active section, which is responsible for optical gain. The inactive section is made of a higher bandgap material than the active section to prevent photon absorption in the phase and grating sections.This allows for altering the density of current injection bearers in the inactive part without interfering with the photon generation of the active part, thus enabling independent control of both the optical gain and wavelength. The schematic of a Distributed Bragg Reflector (DBR) Laser illustrates this concept. The currents applied to each section regulate the laser's operation. The current IA controls the output power of the gain section, while the current IB controls the peak wavelength of the narrowband reflection from the Bragg grating, which closely approximates the Bragg wavelength. By injecting current into the phase section, IP, it is possible to align the longitudinal comb modes with the Bragg peak and finely tune the laser wavelength, resulting in improved SMSR. By simultaneously controlling both the phase and grating sections, it is feasible to achieve continuous tuning ranges of approximately 4 nm and quasi-continuous tuning ranges of approximately 10 nm. In a separate study, a tuning range of 17 nm was achieved using discontinuous tuning. DBR lasers are suitable for many telecom applications as they offer reliability and efficiency with

high output powers and fast tuning times. However, even with their outstanding tuning capabilities, they may still not provide complete C-band coverage.In conclusion, the tuning scope of the DBR will be limited by the ability to tune the Bragg wavelength, which depends on the index alteration in the grate subdivision. When using bearer injection, the limitation is around 5%. The refractive index limitation of wave guides restricts the tuning scope of devices based on index alteration to around 15nm. This is different from the wide addition curve of semiconducting material and EDFA magnifying bandwidth. To achieve broad tunability, it is necessary to change the relative index and wavelength by a multiple of individual wave guide index alteration. Interferometric structures, such as lasers based on MZ interferometry, can offer wider tuning scopes.