WO2005091451A1

WO2005091451A1 - Laser calibration, monitoring and control

Info

Publication number: WO2005091451A1
Application number: PCT/GB2005/001004
Authority: WO
Inventors: Stephen Paul Marsh; Giacinto Busico; Timothy John Simmons; Michael John Wale
Original assignee: Bookham Technology Plc
Priority date: 2004-03-17
Filing date: 2005-03-16
Publication date: 2005-09-29
Also published as: GB0405923D0; GB2412230A

Abstract

A method of determining a correlate of the side mode suppression ratio of a laser comprises measuring the intensity of a beat modulation of the output of a detector arranged to detect light emitted by the laser, the beat modulation being caused by beats between modes of the emission spectrum of the laser, the measured intensity being the correlate.

Description

Laser Calibration. Monitoring and Control

This invention relates to lasers and their electrical control circuits and has particular, but not necessarily exclusive, reference to tuneable lasers, especially for use in telecommunications systems.

Background to the Invention

In this specification the term "light" will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation of any wavelength, especially that having a wavelength between 800 nanometres (nm) and 3000 nm.

Single wavelength lasers are important for a number of applications in optical telecommunications and signal processing applications. These include multiple channel optical telecommunications networks using wavelength division multiplexing (WDM). Such networks can provide advanced features, such as wavelength routing, wavelength conversion, adding and dropping of channels and wavelength manipulation, for example in much the same way as time slot manipulation in time division multiplexed systems. These systems typically operate within the range 1200 to 1650 nm. The principal optical communications wavelength bands are centred on 1300 nm, 1550 nm (C-Band) and 1590 nm (L- Band).

Tuneable lasers for use in such optical communications systems, particularly in connection with the WDM telecommunication systems, are known. A known tuneable laser system comprises stacks of single wavelength distributed Bragg reflector (DBR) lasers, which can be individually selected, or tuned over a narrow wavelength range. Also known are wide tuning range tuneable lasers that can be electronically driven to provide the wavelength required. Limited tuning range tuneable lasers that rely upon thermal effects for tuning are also known. The present invention is particularly (but not exclusively) concerned with multi-section lasers that comprise at least two sections, and in which the wavelength of the optical output is tuneable. An example of such a laser is a three section tuneable laser, as disclosed in Figure 5.5 of "Tunable Laser Diodes" by Amann and Buus (ISBN 0-89006-963-8), which is a wavelength tuneable laser comprising a gain section, a phase change section and a DBR grating. A similar laser is illustrated in Figure 1 of the present specification, and comprises a gain section 1, a phase change section 3 and a DBR grating section 5. The grating section 5 produces a reflection spectrum for light to feedback into the optical cavity of the laser. The reflection spectrum of the grating can be current tuned in wavelength, which is a principal factor in controlling the gross tuning of the emission spectrum of the laser. Fine tuning is controlled using the phase section 3. Further features of the laser are the drive currents applied to respective electrodes on to the gain section 7, phase section 9 and grating section 11, a substrate electrode 13, a front-facet reflective coating 15 and a rear-facet anti-reflection coating 17. The phase section 9 and grating section 11 together comprise the tuning section of the laser.

Other designs of multiple section tuneable laser exist, such as the four section tuneable lasers disclosed by Figure 7.12 of "Tunable Laser Diodes" by Amaann and Buus, and US Patent No. 4,896,325, and the present invention is also concerned with such designs, for example.

Tuneable semiconductor lasers typically comprise either three or four sections, comprising a gain section, a phase section and one or two Distributed Bragg Reflector (DBR) mirrors. Such lasers are also known as DBR lasers. In order to lase, a laser requires an optical cavity, which is formed by two reflectors. In a three section tuneable laser, such as that disclosed in "Tunable Laser Diodes", the first reflector is in the form of a DBR mirror, and the second reflector is provided by a cleaved facet. Such a three section laser is illustrated by Figure 1. In a four section laser such as that referred to above, both reflectors are in the form of DBR mirrors. In operation a tuneable laser may be wavelength stabilised by means of a wavelength locker, which can be either internal or external to the transmitter module. One such wavelength locker is disclosed in UK patent application no. 0227144.3, the entire disclosure of which is incorporated herein by reference.

The wavelength locker samples a small portion of the light generated by the laser, and provides feedback to the electronics that drive the different sections of the laser, typically the phase section. By means of this feedback the laser is kept tuned to a constant wavelength, overcoming those effects of ageing, ambient condition variation and variable power dissipation, which can otherwise lead to wavelength drift.

In DBR lasers, the laser provides an optical cavity for many different optical modes. A problem that can occur in operation is that the amplitude of other modes (known as side modes) besides the lasing mode can become significantly large and interfere with signal clarity. A related problem is that operation of the laser can hop from one mode to another (a mode-hop), interrupting the signal, when the laser is used in a telecommunications transmitter. A typical wavelength locker cannot overcome either of these problems.

The wavelengths to which lasers in telecommunications applications generally are locked are on the International Telecommunications Union's channel spacing grid (ITU-grid), and have spacings such as 100, 50 or potentially 25 GHz.

Wavelength control is applied to the multi-section tuneable laser by means of independently controlling the refractive index properties of some of the sections of the laser, known as the tuning section(s), to satisfy the necessary lasing condition. The tuning section(s) typically comprise(s) the phase section and the at least one distributed Bragg reflector (DBR). It is known to create refractive index changes in tuneable lasers by three main methods. In one method, the free electron plasma effect can be used by free carrier injection, that is, by passing an electric current through the tuning section. In a second method, the fundamental band- gap can be changed by thermal heating. In a third method electro- refraction modification can be brought about using the electro-optic effect. In the latter case, an electrical field is established across the tuning section, which changes the refractive index of the section and thus alters the wavelength of the light as it passes through the section.

The drawings

The background to the present invention, and the invention itself, are described herein, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic diagram of a known three-section DBR grating tuneable laser; Figure 2 is a calibration map graphical representation of the lasing wavelength of the three-section tuneable laser of Figure 1, during incremental tuning, i.e. increasing the drive current in the DBR grating section (the "rear section") first, and then increasing the drive current in the phase section;

Figure 3 is a calibration map graphical representation of the lasing wavelength of the three-section tuneable laser of Figure 1, during decremental tuning, i.e. decreasing the drive current in the DBR grating section (the "rear section") first, and then increasing the drive current in the phase section;

Figure 4 shows the wavelength of the lasing mode of the laser of Figure 1 in an incremental sweep of the DBR grating current at constant phase section current (corresponding to the line marked on Figure 2); Figure 5 shows the wavelength of the lasing mode of the laser of Figure 1 in a corresponding decremental sweep of the DBR grating current (corresponding to the line marked on Figure 3).

Figure 6 is a superposition of the graphs of Figures 4 and 5, illustrating a hysteresis effect in the tuning of the laser of Figure 1;

Figure 7 is a hysteresis map derived from the data of Figures 2 and 3, and shows in black those regions of the calibration map that are affected by hysteresis, and also illustrating exemplary initial channel locations;

Figure 8 shows an enlarged portion of a calibration map for the laser of Figure 1;

Figure 9 is a typical emission spectrum of the laser of Figure 1 operating within a mode (point A of Figure 8); Figure 10 is a typical emission spectrum of the laser of Figure 1 as it approaches a mode boundary (point B of Figure 8);

Figure 11 is a side mode suppression ratio map as measured by a spectrum analyser for a laser according to Figure 1, for an incremental sweep, i.e. increasing the drive current in the DBR grating section (the

"rear section") first, and then increasing the drive current in the phase section;

Figure 12 is a side mode suppression ratio map as measured by a spectrum analyser for a laser according to Figure 1, for a decremental sweep, i.e. decreasing the drive current in the DBR grating section (the

"rear section") first, and then increasing the drive current in the phase section; Figure 13 is a beat modulation power map for a laser according to Figure 1, for an incremental sweep, i.e. increasing the drive current in the DBR grating section (the "rear section") first, and then increasing the drive current in the phase section;

Figure 14 is a beat modulation power map for a laser according to Figure 1, for a decremental sweep, i.e. decreasing the drive current in the DBR grating section (the "rear section") first, and then increasing the drive current in the phase section;

Figure 15 shows the graph of Figure 4 (i.e. for the lasing mode of the laser), and a lower graph of the beat modulation power, for an incremental sweep; Figure 16 shows the graph of Figure 5 (i.e. for the lasing mode of the laser), and a lower graph of the beat modulation power, for a decremental sweep;

Figure 17 shows the upper graphs of the previous two figures superimposed (as per Figure 6), as an upper graph, and the lower graph shows the beat modulation power characteristic for a series of alternate modes (consecutive modes overlap due to the hysteresis, and so for clarity only every third mode has been shown); Figure 18 shows the correlation between the side mode suppression ratio of a laser according to Figure 1, and the power of the beat modulation;

Figure 19 is a schematic calibration map of the laser of Figure 1, showing optimised locking lines for each channel, with the channels located at positions of the correct wavelength on those lines, situated between hysteresis regions; and Figure 20 is a schematic diagram of an embodiment of an apparatus according to the invention.

The accompanying Figure 2 illustrates a calibration map, showing a graphical representation of the lasing wavelength of the three section tuneable laser of Figure 1, during incremental tuning (increasing current in the DBR and phase section), and is similar to that disclosed in Figure 9.1 of "Tunable Laser Diodes" by Amann and Buus. It illustrates the relationship between the lasing wavelength of the laser and the currents applied to the phase section and the DBR. By controlling the currents to the phase section and the DBR, the laser can be tuned across the whole of its wavelength range. Figure 3 shows a corresponding calibration map for decremental tuning (decreasing current in the DBR and increasing the current in the phase section). The two figures are slightly different due to a hysteresis effect that occurs in the tuning of the laser.

Figure 4 shows the wavelength of the lasing mode of the laser in an incremental sweep of the DBR current at constant phase section current (corresponding to the line marked on Figure 2). Figure 5 shows the wavelength of the lasing mode of the laser in a corresponding decremental sweep of the DBR current (line on Figure 3). The hysteresis effect in the tuning is illustrated by Figure 6, in which the graphs of the two previous figures are superimposed.

In tuning tuneable lasers, such as that disclosed in Figure 1, incremental and decremental tuning of the phase section and DBR drive currents result in different mode boundaries, due to the hysteresis effect. This difference is illustrated by comparing Figure 3, which shows decremental tuning, with Figure 2, which shows incremental tuning over the same range of drive currents. The hysteresis is further illustrated by Figure 6. In this figure the black graph shows the wavelength change when the DBR (Year section') current is swept up (incremental current) in current along the line marked on Figure 2, and the grey graph shows the wavelength output of the laser when the DBR current is swept down (decremental tuning) along the comparable line on Figure 3.

The operating characteristics of the laser in Figure 1 are determined by a combination of the effects of the reflection spectra of the DBR and the end facet, the attenuation properties of each section of the laser, the gain properties of the gain section, and the tuning direction.

However, the tuning response of the laser is principally determined by the tuning of the DBR grating. The DBR grating of the laser in Figure 1 is of a single constant pitch and produces a reflection spectrum comprising substantially a single peak. This single reflection peak can be tuned in wavelength by means of the refractive index change techniques described previously, and provides the method for gross tuning control. Tuning of the phase section provides fine tuning control.

As is apparent from Figure 2, the calibration map for such a three section tuneable laser as has been described typically comprises a pattern of stripes, each of which corresponds to a single optical mode of the laser's optical cavity. To better understand this invention it is useful to explain what is meant by the terms optical cavity and modes in the context of this specification.

The physical length of the optical cavity of the laser of Figure 1 is defined at one end by a cleaved end facet 7. However, at the other end the reflection of the light is distributed along the length of the DBR grating section 5. The relationship between the amplitude of the light reflected back to the input end of the grating section 5 from different points along the length of the grating section and the distance from the light input end of the grating section is a decay function. The decay has a characteristic penetration depth which is defined by the optical path length, from the input end of the grating section 5, to the point at which the amplitude of the light that is reflected back to the input end has fallen to a particular proportion of the intensity of the input light. The penetration depth is principally a function of grating strength.

The optical cavity is considered to be defined between the cleaved facet 7 and the location of the penetration depth point of the grating section 5. Due to the finite length of the penetration depth of the grating, the reflection spectrum of the grating produces a principal peak of finite width. The laser structure forms an optical cavity for a range of discrete wavelengths that correspond to an integer number of unit standing-wave periods. However, since each wavelength experiences an optical cavity of approximately the same optical path length, the number of unit standing- wave periods that can be supported within it is different for each wavelength, and each is termed a separate optical mode. The number of standing waves that can be supported within the optical path length of the optical cavity is given by relationship:

N = 2n_effL /λ

where n_eff is the effective refractive index of the optical cavity, L is the physical length of the optical cavity and λ is the wavelength of the mode.

When the laser lases there is one principal wavelength of light emitted by the laser. However, the gain section 1 generates light over a range of wavelengths and as a consequence of the non-zero width of the principal peak of the reflection spectrum of the grating, positive feedback of light of several consecutive modes occurs within the cavity and is emitted from the laser in the form of an emission spectrum. Figure 9 shows an exemplary emission spectrum of a three section laser according to Figure 1.

In operation, it is desirable that the laser emission spectrum should be dominated by lasing at a single lasing mode that is held fixed at a predetermined wavelength. However, other modes will also be generated by the laser and appear in the emission spectrum. These other modes are known as side modes, and the ratio of their intensity relative to the lasing mode is a commonly measured property of a laser. The term side mode suppression ratio (SMSR) is used to describe the ratio between the intensity of the lasing mode and that of the highest intensity side mode in the emission spectrum, which is typically one of the two side modes that are consecutive to the lasing mode (i.e. immediately sequential to the lasing mode in the emission spectrum).

It is important in telecommunications applications to optimise the locations of the channels within the calibration map, with respect to the SMSR value, when it is used as part of an optical transmitter, to ensure that the emission spectrum has acceptable levels of SMSR during its operating lifetime, and does not impact upon the system performance when the laser is used in the transmission of encoded data.

Performing such a calibration requires that the tuneable laser is characterised across its whole operating range, typically by measuring the power, principal wavelength and SMSR value of the emission spectrum at an array of points across the whole of the calibration map. The accompanying Figures 11 and 12 show the SMSR map for the tuneable laser shown in Figure 1, measured across the same DBR grating section and phase section current ranges as for the calibration maps of Figures 2 and 3. However, SMSR calibration across the whole of the control map is extremely time consuming, since each data point requires an expensive spectrum analyser to perform a sweep across the emission spectrum at each point in a dense array of points across the calibration map, in order to determine the intensity of the second largest mode at each point in the array. Such a calibration of a wide band tuneable laser may take many hours.

The lasers are typically preset for operation at specific locations on the calibration map, known as channels (on the ITU grid, for telecommunications applications). The measurement of the SMSR (Figures 11 and 12) across the whole calibration map enables the optimum operating point to be determined for each channel, as shown in Figure 19. In prior art applications the locations of the channels are typically chosen to be away from the mode boundaries of the calibration map, so as to enable a limited amount of re-tuning of the phase section current by the wavelength locker system for wavelength stabilisation, whilst remaining within the same mode.

It is preferable for a laser that exhibits hysteresis, such as that in Figure 1, that the optimum channel locations are chosen in regions of the control map that are free of hysteresis. Figure 7 is derived from the data in Figures 2 and 3, and shows in black those regions of the calibration map in which hysteresis occurs. Figure 7 also shows exemplary locations in which the channels are initially calibrated (see also schematic Figure 19).

As well as determining the SMSR value of the emission spectrum at the initial channel locations, the prior art calibration process must also take into account the SMSR value within the re-tuning range ('wavelength locker operating range') of each channel, to ensure that that also is of an acceptable level. However, in order to meet the SMSR requirement across the re-tuning range of a wavelength locker, the locations of the initial channels may not necessarily be chosen at positions of maximal SMSR value.

The variation of the SMSR value at different locations within a mode is illustrated by some of the figures. Figure 8 shows a schematic illustration of an enlarged portion of a calibration map. The letter A marks an optimum channel location in one of the strip like mode regions for a particular wavelength, which is distant from the mode boundaries. At an optimised channel location, such as point A, the side modes are small, and consequently the SMSR values are high, as is shown in Figure 9. The letter B marks another point within the same mode at which the lasing mode has the same wavelength, and which is close to the boundary between that mode and an adjoining mode. At points such as B, close to the mode boundary, typically one of the consecutive side modes is larger than at point A, and consequently the SMSR values are smaller, as is shown in Figure 10.

It is known to those skilled in the art that several factors affect the performance of a laser, especially including ageing. These factors can result in drifting of the laser modes with respect to the DBR grating section and phase section currents. A further effect of ageing is that the amount by which a laser tuning section tunes for a given change in drive current, which is known as tuning efficiency, may decrease with age.

In the initial calibration of a laser the locations of the channels within the calibration map are generally chosen to optimise the range over which the wavelength locker can safely re-tune the phase section current without reaching a mode boundary. A consequence of the performance effects referred to above is that a danger arises that the operating positions of the channels can become close to mode boundaries creating the risk of mode-hopping. In particular, due to drifting of the modes on the calibration map a channel can move to the edge of a mode, and then when feedback from the wavelength locker system is used to correct for wavelength drift, by adjusting the phase section current, it is possible that the laser could mode-hop over a boundary into the next mode.

In telecommunications applications, tuneable lasers, such as that shown in Figure 1, are often used as components inside optical transmitter modules. In use data is encoded onto the light emitted from the laser, typically by use of a separate modulator, such as is disclosed by WO01/77741, the entire disclosure of which is incorporated herein by reference. (Direct modulation of lasers is also known in the art.) Such applications typically involve simultaneous transmission of many signals along each optical fibre, and require extremely high reliability of the received data when decoded (typically error rates are less than 1 in 10¹¹ in data that has been recovered using error correction bits). The data reception can be jeopardised by the laser operating close to a mode-hop boundary, which could result in cross-talk/adjacent channel interference. Yet more damagingly, the data connection would be broken in the event that the operation of the laser should move across the boundary into another mode (known as a 'mode-hop'), and cease to be locked to the wavelength of the channel. Also, subject to the relative size of the channel and mode spacings, a mode-hop could potentially result in the laser hopping onto an adjacent channel and thus creating further problems.

In the prior art all known simple methods for determining how the modes of a typical tuneable telecommunications laser have drifted have significant weaknesses, and typically a laser would require to be fully recalibrated. However, recalibration of tuneable lasers is not common, since it is time consuming, requires expensive and bulky equipment, and typically cannot be performed in-situ, once a device has been deployed in a network.

Some prior art techniques that are suitable for in-situ use, but which are not suitable for telecommunications lasers are known and are described below.

S. L. Woodward et al. in "The side-mode-suppression ratio of a tunable DBR laser" IEEE Photon. Technol. Lett., vol 2, pp 854-856, 1990, discloses a correlation between the optical output power of a laser and the position within the operating region in which a particular mode is dominant (known as "the position within the mode"). The paper proposes a predictive technique for determining a suitable locking point from measurement of the locations of the mode boundaries. However, as the paper concedes, the predicted value and SMSR maximum do not coincide, even in the described device.

S. L. Woodward et al. in "A control loop which ensures high side-mode- suppression ratio in a tunable DBR laser" IEEE Photon. Technol. Lett., vol 4, pp 417-419, 1992, discloses a technique for correlating the output light transmitted through a DBR laser section with DBR current, and locking to the output power maximum. The technique does not determine the SMSR maximum, and the correlation between the output power and the SMSR maximum is affected by several factors, and not just the DBR current.

H. Ishii et al. in "Wavelength stabilization of a three-electrode distributed Bragg reflector laser with longitudinal mode control" Electron. Lett., vol 33, pp 494-496, 1997, discloses a technique for modulating the phase section current of a tuneable laser to lock to the output power maximum. As with the earlier paper by Woodward, the technique does not determine the SMSR maximum, and the correlation between the output power and the SMSR maximum is affected by several factors, and not just the phase section current.

G. Sarlet et al. in "Wavelength and Mode Stabilization of Widely Tunable SG-DBR and SSG-DBR Lasers" IEEE Photonics Technology Letters, Vol 11, No 11, November 1999, discloses a correlation between the voltage to the active (gain) section of the laser and the location of the operating point within a mode, with respect to the drive currents of the tuning section of a four section tuneable laser. This technique does not disclose locking the operation of the laser to the local SMSR maximum.

All four of the above prior art methods assume a fixed relationship between output power and the position of the SMSR maxima within the modes. However, in instances of practical importance this assumption may be invalid.

The present invention

According to a first aspect, the present invention provides a method of determining a correlate of the side mode suppression ratio of a laser, comprising measuring the intensity of a beat modulation of the output of a detector arranged to detect light emitted by the laser, the beat modulation being caused by beats between modes of the emission spectrum of the laser, the measured intensity being the correlate. The beat modulation of which the intensity is measured preferably is caused by beats between modes of the emission spectrum of the laser that are immediately sequential in the emission spectrum. This is because, as mentioned above, the side mode suppression ratio (SMSR) is the ratio between the intensity of the lasing mode and that of the highest intensity side mode in the emission spectrum, which is typically one of the two side modes that are immediately sequential to the lasing mode in the emission spectrum. Consequently, measuring the intensity of the beat modulation caused by beats between immediately sequential modes of the emission spectrum of the laser normally obtains the best correlation with the SMSR (using the method of the invention). However, it is sometimes the case that the highest intensity side mode is not immediately sequential to the lasing mode in the emission spectrum. Consequently, the invention also encompasses the possibility that the beat modulation of which the intensity is measured, is caused by beats of a particular frequency between modes of the emission spectrum of the laser that are not immediately sequential in the emission spectrum.

It will be understood by the skilled person that the measured beat modulation is caused by beats between substantially all such modes of the emission spectrum of the laser (i.e. either substantially all immediately sequential modes, or substantially all non-immediately sequential modes that beat at a particular frequency). Consequently, the skilled person will understand that the correlate of the side mode suppression ratio that is determined by the present invention is only approximate. However, the skilled person will also appreciate that due to the typical dominance of the emission spectrum by the lasing mode, and the typical dominance of the side modes by a single side mode, the correlate of the SMSR determined by the present invention will typically be a very good approximation. The method according to the invention preferably comprises determining a variation of the correlate of the side mode suppression ratio of the laser with position within the lasing mode, by varying an electrical drive current or voltage of the laser and measuring a variation in the intensity of the beat modulation of the output of the detector.

Advantageously, the method may comprise determining a position within the lasing mode at which the correlate of the side mode suppression ratio of the laser is a maximum, by determining a position within the lasing mode at which the intensity of the beat modulation of the output of the detector is a minimum.

Preferably, the variation of the electrical drive current or voltage of the laser is carried out by applying a dither modulation to the electrical drive current or voltage of the laser (e.g. a drive current of a phase change or grating section of a multi-section laser).

In some preferred embodiments of the invention, the method may be used to calibrate a laser such that the correlate of the side mode suppression ratio of the laser is substantially known for a range of control conditions of the laser.

A second aspect of the invention provides a method of providing a warning of wavelength drift within the lasing mode of a laser, of light emitted by the laser, comprising determining a correlate of the side mode suppression ratio of the laser by a method according to the first aspect of the invention, and providing an alert which triggers at a preset value of the correlate.

A third aspect of the invention provides a method of controlling the side mode suppression ratio of a laser, by determining the correlate of the side mode suppression ratio by a method according to the first aspect of the invention, and controlling the laser in dependence upon the determined correlate. A fourth aspect of the invention provides a method of preventing mode- hopping of a laser, comprising controlling the side mode suppression ratio of the laser according to the third aspect of the invention, by preventing the correlate of the side mode suppression ratio from increasing above a preset value.

The methods of the invention preferably include using a control device arranged to control the laser automatically in dependence upon the determined correlate.

In many embodiments of the invention, it is not necessary actually to determine the side mode suppression ratio of the laser, but merely to use the correlate, i.e. the measured intensity of the beat modulation of the output of the detector. However, the invention does not preclude determining the actual side mode suppression ratio of the laser, and many embodiments of the invention may do so, from the measured intensity of the beat modulation.

The methods according to the invention may, in some embodiments, be performed on a laser installed in a telecommunications network, sensor network or data network. Alternatively, the methods may be performed on a laser as part of a manufacturing process of the laser or a device of which the laser is a component part (for example a telecommunications transmitter).

A fifth aspect of the invention provides an apparatus for determining a correlate of the side mode suppression ratio of a laser, comprising a detector for detecting light emitted by the laser and a measuring device adapted to measure the intensity of a beat modulation of the output of the detector, the beat modulation being caused by beats between modes of the emission spectrum of the laser, the measured intensity being the correlate. The beat modulation that is measured by the measuring device of the apparatus preferably is a beat modulation as described above in relation to the methods of the invention.

Preferably the apparatus is adapted to monitor variations in the correlate of the side mode suppression ratio of the laser, by measuring variations in the intensity of the beat modulation of the output of the detector when the wavelength of light emitted by the laser is varied.

More preferably, the apparatus may be adapted to determine a control condition of the laser at which the side mode suppression ratio of the laser is a maximum, by determining a control condition at which the intensity of the beat modulation of the output of the detector is a minimum.

The apparatus according to the fifth aspect of the invention advantageously may be adapted to calibrate a laser such that the correlate of the side mode suppression ratio of the laser is known for a range of control conditions of the laser.

Similarly to the methods according to the invention, the apparatus may be adapted to determine the actual side mode suppression ratio of the laser, from the correlate.

A sixth aspect of the invention provides an apparatus for providing a warning of wavelength drift within the lasing mode of a laser, of light emitted by the laser, comprising an apparatus according to the fifth aspect of the invention, including an alert which triggers at a preset value of the determined correlate.

A seventh aspect of the invention provides an apparatus for controlling the side mode suppression ratio of a laser, comprising an apparatus according to the fifth or sixth aspects of the invention, including a control device adapted to control the emission spectrum of the laser in accordance with measurements made by the measuring device. An eighth aspect of the invention provides apparatus for preventing mode- hopping of a laser, comprising an apparatus according to the invention, adapted to prevent the correlate of the side mode suppression ratio from falling below a preset value.

With the apparatus or method of the invention, the correlate may used to tune the laser without causing mode-hoping of the output of the laser. For example, the correlate may be used to tune the laser between operating channels within the same lasing mode of the laser.

In some preferred embodiments of the invention, the apparatus is portable. For example, the apparatus may be adapted to operate on the light from a laser, either directly or through a network fibre, installed in a telecommunications network.

The apparatus may comprise a component of a larger apparatus for carrying out a plurality of types of measurements on the laser or the light emitted therefrom. For example, the larger apparatus may include a spectrum analyser.

The lasers referred to herein may be tuneable lasers, and varying the wavelength of light emitted by a laser and/or controlling a laser (as described herein) consequently may comprise tuning a tuneable laser.

The present invention advantageously provides (among other things) a means for in-situ recalibration of tuneable lasers using the RF power of the inter-mode beat frequency, to determine the side-mode suppression ration (SMSR) for use in a feedback system.

Furthermore, the invention may be applied to any laser, which is capable of operating on more than a single mode, in which the optimum single mode/side mode suppression ratio is achieved by a tuning element within the cavity under electronic control (e.g. a gas laser in which optimum tuning is achieved by adjustment of an additional mirror in the coupled cavity, or a solid state laser).

The apparatus of the invention may be incorporated into a system such that it can provide mode control to a plurality of lasers.

Preferred and other optional features of the invention are described below (including with reference to the accompanying figures) and in the dependent claims.

If the light emitted by a laser is measured by a square law optical power detector (for example by a photodiode), the detected signal will be in proportion to the square of the sum of the electric fields of the light of the different wavelengths in the emission spectrum of the laser. Light of a wavelength close to the principal wavelength of the laser will produce a characteristic beat modulation in the amplitude of the detected signal. The beat frequency of a tuneable laser is dependent upon mode spacing size (i.e. the wavelength separation between modes), which is dependent upon the optical cavity length of the laser, and is typically at an RF (radio frequency). In the case of preferred lasers for use according to the invention, for example the design shown in Figure 1, this frequency is approximately 55GHz. The power of this side mode beat modulation is known as the RF beat power.

As has been mentioned above, away from the position within a mode at which the SMSR value is highest, one of the side modes, typically one adjacent to the lasing mode, becomes larger than the other side modes, which can be considered to be small. Consequently the beat modulation is dominated by an interaction, at the detector, of the light at the wavelengths of the lasing mode and largest side mode. The intensity of the beat modulation is a therefore an approximate function of the relative intensities of the lasing mode and the highest intensity side mode, and consequently measurements of the intensity of the beat modulation can be used to determine an approximation (but a good approximation) of the SMSR of the emission spectrum of the laser. Figure 18 shows the correlation between the SMSR value and the power of the beat modulation. Figure 13 shows the beat power map for incremental tuning, corresponding to the calibration map in Figure 2. Figure 14 shows a corresponding beat power map for decremental tuning.

The lower graph of Figure 15 shows a stylised illustration of the RF beat power of a laser in an incremental sweep of the DBR current, and the upper graph shows the wavelength of the lasing mode. This sweep corresponds to the line of constant phase current marked on Figure 2. The graphs in Figure 16 show the corresponding values for a decremental sweep of the DBR current. The upper graph of Figure 17 shows the upper graphs of the previous two figures superimposed (as per Figure 6). The lower graph in Figure 17 shows a stylised illustration of the RF beat power characteristic for a series of alternate modes (consecutive modes overlap due to the hysteresis, and so for clarity only alternate modes have been shown).

A photodetector may detect both the DC intensity of the light (which is used for monitoring the output level of the optical power of the laser) and the RF beat power. This enables it to be used to monitor drifting of the calibration map, which can in turn be used to update the control electronics (often known as the control shell) that drives the laser. In particular, measurement of the RF beat power can be used in a feedback system to maintain the laser at a fixed position within a mode. Typically this may be by making adjustments to the DBR grating section (the Year section') current to complement the fine wavelength tuning of the phase section that is applied by the wavelength locker feedback system.

Conveniently the laser may be maintained at the position within the mode at which the RF beat power is at a minimum. However, it could also be possible to lock to other points within each mode. Advantageously a relatively low frequency modulation (known as dither, and preferably in the range lkHz-lMHz) may be applied to the DBR drive current. This can be monitored to detect an RF beat power minimum and to maintain the laser at that location within a mode. Potentially dither could alternatively or also be applied to other sections of the laser (e.g. at least one other tuning section).

The invention has numerous advantages, some of which are as follows.

A first advantage of the invention is "active mode-hop prevention". By maintaining the laser at a particular position within a mode, the present invention prevents the possibility of the laser experiencing a mode-hop (the severe problems of which have been described above).

A second advantage is an ability to lock to the SMSR maximum (for a chosen channel wavelength) for optimised signal clarity and to overcome deleterious effect such as ageing. In the preferred method, the present invention enables a tuneable laser to lock to both the location of a local RF beat power minimum (with respect to the tuning section currents) and to the operating wavelength of a channel. Consequently the operating locations of the laser can be maintained on a line within a mode defined by the locations at which the RF beat power is a minimum for each wavelength (which could be described as 'an optimised locking line' - this is not a pre-existing term). Thus the laser will always (in this scenario) be operated at approximately the maximum SMSR value for a wavelength (due to the high degree of correlation between SMSR and RF beat power), thereby optimising the received signal clarity when the laser is used in telecommunications applications. Thus by tracking the optimised locking line, the present invention enables the laser to continue to operate at an SMSR maximum despite deleterious effects of ageing and ambient conditions to be overcome. Figure 19 is a schematic calibration map of the laser of Figure 1, showing optimised locking lines 101 for each channel 103, with the channels located at positions of the correct wavelength on those lines, situated between hysteresis regions 105. A third advantage of the invention is that it enables optimised calibration of initial channels. As well as determining the SMSR value of the emission spectrum at an initial channel location, the prior art calibration process must also take into account the SMSR value within the re-tuning range of a channel, to ensure that that also is of an acceptable level. However, in order to meet the SMSR requirement across the whole range, in the prior art it is possible that the initial channel locations may need to be located off the optimised locking line, and so away from the SMSR maximum for the wavelength. In contrast the preferred method of the present invention enables the channels always to be located at the SMSR maximum for that wavelength.

A fourth advantage is increased operational lifetime. Being able to guarantee the operation of the laser against the effects of ageing can increase the operational lifetime that is available to the laser.

A fifth advantage is yield improvement. As has been described above, the design of prior art laser control maps must allow for a phase section current re-tuning range, which is used by a wavelength locker to provide wavelength stabilisation of the channel, and the need for the SMSR to be at an acceptable level across the whole range. In contrast the present invention normally requires only that the SMSR level is at an acceptable level at a line of potential locking points within a mode. Consequently the present invention is much less demanding of the SMSR characteristics of the laser, which could have a positive impact upon yield in industrial laser manufacture.

A sixth advantage is locking in the hysteresis region. In the prior art it has been necessary to chose the initial channel locations on the control map to be in the hysteresis-free regions of the control map and at such a location that the corresponding wavelength locker operating range also does not extend into any regions of hysteresis. In contrast, the present invention is capable of locking to channel locations that are within regions of hysteresis on the laser control map. (Typically the RF beat power will be in a hysteresis free region, but this could be useful if the laser were locked to a point other than the RF beat power minimum).

A seventh advantage is a combined benefit of possible reduced cost and higher speed of calibration. The present invention enables a laser to be fully characterised with respect to spectral purity at higher speed (e.g. a few minutes) and at lower cost than presently known techniques (e.g. a spectrum analyser, which typically requires many hours and is costly, or by other direct wavelength measurement techniques, which only provide information about the wavelength, and not the SMSR). Alternatively the invention could be combined with a spectrum analyser to enable targeting of the analysis to only those portions of the calibration map that are of interest.

Figure 20 is a schematic diagram of an embodiment of an apparatus according to the invention. The apparatus 201 comprises an optical detector diode (i.e. a photodiode) 203 arranged to sample a proportion of the light emitted by a laser 205 (as shown, a laser diode). The sampled proportion of the laser light may be sampled by means of a beamsplitter 207, for example. The photodiode 203 is selected to have a bandwidth which encompasses the frequency of the beat modulation caused by beating between immediately sequential modes of the laser emission spectrum (i.e. beating between the lasing mode and the immediately adjacent side mode, and between immediately adjacent side modes). For the preferred optical transmission lasers, the frequency of the beat modulation is generally in the radio frequency range, especially in the GHz range, for example approximately 55 GHz.

As shown in Figure 20, the apparatus preferably includes an amplifier 19 tuned to the beat frequency (preferably a band pass low noise amplifier), the apparatus also includes a measuring device 211 adapted to measure the intensity (i.e. power) of the beat modulation of the output of the photodiode 203. The measuring device 211 comprises a radio frequency detector diode having a bandwidth encompassing the beat frequency. The measured intensity of the beat modulation is then fed back via electronics 215 to a control device 213 that controls (i.e. tunes) the laser 205 in response to the measured intensity of the beat modulation. Alternatively the apparatus could also include a down-converter to convert the output of the amplifier 209 or the photodiode 203 to a convenient lower frequency, as is well known in the art. The apparatus of Figure 20 also includes a further beamsplitter 217 and a wavelength locker 219.

Many applications and embodiments of the present invention are envisaged. For example, in one embodiment, an apparatus according to the invention is incorporated into a device, e.g. a telecommunications transmitter, that uses a tuneable laser, to provide in-situ control throughout the lifetime of the device, following deployment in an optical network. Modes of operation include: real-time maintenance of a single channel; short time-scale re-calibration of the full control map, either when not in use, or by periodic (e.g. monthly) shut-down of the device. The apparatus may be either co-packaged with the transmitter (or other device), or it may be a separately packaged apparatus (e.g. 'pig-tailed' with an optical fibre input and output, so it can be used in series with the transmitter or other device).

A further application of an apparatus according to the invention is in a portable apparatus for measurement the RF beat power of lasers, which could have particular advantages for enabling characteristics of lasers that have been deployed into networks to be measured in the operational environment, for example. The portable apparatus could further enable the re-calibration of such lasers. Typically the measurements could be made by connecting the portable apparatus to a fibre along which the laser is transmitting or by connecting directly to a transmitter, for example.

A yet further application of such an apparatus according to the invention is for use in laboratories and manufacturing facilities to provide a high speed calibration device. Such an apparatus could either be a stand-alone unit, or it could be a composite unit containing other measurement systems, e.g. a spectrum analyser. Such a stand-alone unit could offer a considerable price reduction compared with the purchase of a spectrum analyser, for example. Such a composite unit could enable a reduction in the time necessary to obtain the useful data from an SMSR calibration, but using the results obtained from a measurement with the apparatus according to the invention, to restrict the scope of the SMSR calibration to only the regions of the calibration map that are of relevance in producing the control map, for example.

Although this invention has been described primarily in terms of a three section DBR grating laser, it is not restricted to such lasers, but can be applied to other lasers, for example other multi-section lasers, such as four section DBR grating lasers.

US Patent No. 4,896,325 describes a four section tuneable laser that tunes by a Vernier tuning method, in which two DBR grating sections are used, each of which produces a comb-like reflection spectrum, but in which the spacings between the peaks of the comb-like spectra differ, such that only one pair of peaks can have the same wavelength for any particular DBR grating drive current. Separate tuning of the DBR grating sections can be used to control which peaks coincide, at which frequency the laser exceeds the lasing threshold, and lases. In such a Vernier tuning laser, the gross tuning is provided by the two DBR grating sections, and the fine tuning is provided by the phase section. In the case that a wavelength locker system is provided in such a laser device, with feedback to the phase section, then the present invention could provide a further feedback system that provides additional control to the two DBR grating sections such that they maintain the laser at a particular position within the lasing mode for a given wavelength, and said position may be such that the correlate of side mode suppression ratio is a maximum. Further, the additional control could be used to control the relative tuning of the two DBR grating sections such that they tune together in wavelength.

International patent application WO 03/012936 discloses an alternative four section digital super-mode DBR laser (DS-DBR) in which a lasing cavity is formed between a phase-shifted DBR grating, that produces a uniform comb-like reflection spectrum of narrow peaks, and a segmented DBR grating. The segmented DBR section comprises segments of different pitches, which produces a comb of broad peaks, such that no wavelength in the laser is above the lasing threshold, in its un-driven state. Each of the segments is driven by a separate drive current applied through a separate electrode, and when one peak is re-tuned to coincide with another peak, then the laser reaches the lasing threshold at one wavelength within the broad reinforced peak. The lasing wavelength that is determined by a narrow peak of the phase-shifted DBR coincides with the broad reinforced peak. In such a DS-DBR laser, in the case that a wavelength locker system provides feedback to the phase section, the present invention could provide a further feedback system that provides additional control to at least the phase-shifted DBR grating.

Although the invention has been described primarily in the context of continuous wave (CW) lasers, which in telecommunications applications are typically used with separate modulators, it will be apparent to one skilled in the art that it can also be applied to directly modulated lasers (in which, effectively, the laser output is turned on and off to encode the data signal).

Claims

1. A method of determining a correlate of the side mode suppression ratio of a laser, comprising measuring the intensity of a beat modulation of the output of a detector arranged to detect light emitted by the laser, the beat modulation being caused by beats between modes of the emission spectrum of the laser, the measured intensity being the correlate.

2. A method according to claim 1, in which the beat modulation is caused by beats between modes of the emission spectrum of the laser that are immediately sequential in the emission spectrum.

3. A method according to claim 1, in which the beat modulation is caused by beats of a particular frequency between modes of the emission spectrum of the laser that are not immediately sequential in the emission spectrum.

4. A method according to claim 2 or claim 3, in which the beat modulation is caused by beats between substantially all such modes of the emission spectrum of the laser.

5. A method according to any preceding claim, comprising determining a variation of the correlate of the side mode suppression ratio of the laser with position within the lasing mode, by varying an electrical drive current or voltage of the laser and measuring a variation in the intensity of the beat modulation of the output of the detector.

6. A method according to claim 5, comprising determining a position within the lasing mode at which the correlate of the side mode suppression ratio of the laser is a maximum, by determining a position within the lasing mode at which the intensity of the beat modulation of the output of the detector is a minimum.

7. A method according to claim 5 or claim 6, in which the varying of the electrical drive current or voltage of the laser is carried out by applying a dither modulation to the electrical drive current or voltage of the laser.

8. A method according to any preceding claim, used to calibrate a laser such that the correlate of the side mode suppression ratio of the laser is substantially known for a range of control conditions of the laser.

9. A method of providing a warning of wavelength drift within the lasing mode of a laser, of light emitted by the laser, comprising determining a correlate of the side mode suppression ratio of the laser by a method according to any preceding claim, and providing an alert which triggers at a preset value of the correlate.

10. A method of controlling the side mode suppression ratio of a laser, by determining the correlate of the side mode suppression ratio by a method according to any preceding claim, and controlling the laser in dependence upon the determined correlate.

11. A method of preventing mode-hopping of a laser, comprising controlling the side mode suppression ratio of the laser according to claim 10 by preventing the correlate of the side mode suppression ratio from increasing above a preset value.

12. A method according to any preceding claim, including using a control device arranged to control the laser automatically in dependence upon the determined correlate.

13. A method according to any preceding claim, including the step of determining the actual side mode suppression ratio of the laser, from the correlate.

14. A method according to any preceding claim, performed on a laser installed in a telecommunications network, sensor network or data network.

15. A method according to any one of claims 1 to 13, performed on a laser as part of a manufacturing process of the laser or a device of which the laser is a component part.

16. A method according to claim 15, in which the device comprises a telecommunications transmitter or source element in a data or sensor network.

17. An apparatus for determining a correlate of the side mode suppression ratio of a laser, comprising a detector for detecting light emitted by the laser and a measuring device adapted to measure the intensity of a beat modulation of the output of the detector, the beat modulation being caused by beats between modes of the emission spectrum of the laser, the measured intensity being the correlate.

18. An apparatus according to claim 17, in which the beat modulation is caused by beats between modes of the emission spectrum of the laser that are immediately sequential in the emission spectrum.

19. An apparatus according to claim 17, in which the beat modulation is caused by beats of a particular frequency between modes of the emission spectrum of the laser that are not immediately sequential in the emission spectrum.

20. An apparatus according to claim 18 or claim 19, in which the beat modulation is caused by beats between substantially all such modes of the emission spectrum of the laser.

21. An apparatus according to any one of claims 17 to 20, adapted to monitor variations in the correlate of the side mode suppression ratio of a laser, by measuring variations in the intensity of the beat modulation of the output of the detector when an electrical drive current or voltage of the laser is varied.

22. An apparatus according to claim 21, adapted to determine a control condition of a laser at which the side mode suppression ratio of the laser is a maximum, by determining a control condition at which the intensity of the beat modulation of the output of the detector is a minimum.

23. An apparatus according to any one of claims 17 to 22, adapted to calibrate a laser such that the correlate of the side mode suppression ratio of the laser is known for a range of control conditions of the laser.

24. An apparatus according to any one of claims 17 to 23, adapted to determine the actual side mode suppression ratio of the laser, from the correlate.

25. An apparatus for providing a warning of wavelength drift within the lasing mode of a laser, of light emitted by the laser, comprising an apparatus according to any one of claims 17 to 24, including an alert which triggers at a preset value of the determined correlate.

26. An apparatus for controlling the side mode suppression ratio of a laser, comprising an apparatus according to any one of claims 17 to 25, including a control device adapted to control the emission spectrum of the laser in accordance with measurements made by the measuring device.

27. An apparatus for preventing mode-hopping of a laser, comprising an apparatus according to claim 26, adapted to prevent the correlate of the side mode suppression ratio from falling below a preset value.

28. An apparatus according to any one of claims 17 to 27, which is portable.

29. An apparatus according to any one of claims 17 to 28, which is adapted to operate on a laser installed in a telecommunications network.

30. An apparatus according to any one of claims 17 to 29, comprising a component of a larger apparatus for carrying out a plurality of types of measurements on the laser or the light emitted therefrom.

31. An apparatus according to claim 30, in which the larger apparatus includes a spectrum analyser.

32. A method or apparatus according to any preceding claim, in which the laser is a tuneable laser.

33. A method or apparatus according to claim 32, in which varying of the wavelength of light emitted by the laser and/or controlling the laser, comprise tuning of the laser.

34. A method or apparatus according to claim 32 or claim 33, in which the laser comprises a plurality of tuneable sections, and in which the determined correlate of the side mode suppression ratio of the laser is used to control relative tuning of the tuneable sections of the laser.

35. A method or apparatus according to any one of claims 32 to 34, in which the correlate is used to tune the laser without causing mode-hoping of the output of the laser.

36. A method or apparatus according to claim 35, in which the correlate is used to tune the laser between operating channels within the same lasing mode of the laser.

37. A method or apparatus according to any preceding claim, in which the laser is a semiconductor laser.

38. A method or apparatus according to any one of claims 1 to 36, in which the laser is a gas laser.

39. A method or apparatus according to any one of claims 1 to 36, in which the laser is a solid state laser.

40. A system comprising a plurality of lasers and an apparatus according to any one of claims 17 to 39, wherein the apparatus is arranged to provide mode control to the lasers of the system.