WO2011043718A1

WO2011043718A1 - Method and device for controlling the operation point of a semiconductor mach-zender modulator

Info

Publication number: WO2011043718A1
Application number: PCT/SE2010/051063
Authority: WO
Inventors: Dave Adams; Edgard Goobar; Robert LEWÉN
Original assignee: Syntune Ab
Priority date: 2009-10-05
Filing date: 2010-10-01
Publication date: 2011-04-14

Abstract

Method for controlling the operation point of a semiconductor Mach Zehnder Modulator (MZM) (2), the MZM (2) comprising an input waveguide (2a) which is divided into two intermediate waveguides (2b, 2c) and then joined together in an output waveguide (2d,2e), in which method the input power from a laser (1) into the MZM (2) is stabilised and the operation point of the MZM (2) is then controlled given the stabilised input from the laser (1). The invention is charachterised in that the laser (1) and the MZM (2) are caused to be monolithically integrated or hybrid integrated, in that the photocurrent in at least one of said input- and/or intermediate waveguides (2a, 2b, 2c) of the MZM (2) is measured and in that the measured value is fed to a control circuit (4), which control circuit (4) is caused to stabilize the input light power fed into the MZM (2) based upon the measured photocurrent value. The invention also relates to an integrated transmitter.

Description

METHOD AND DEVICE FOR CONTROLLING THE OPERATION POINT OF A

SEMICONDUCTOR MACH-ZENDER MODULATOR

The present invention relates to a method and a device for controlling the operation point of a semiconductor Mach- Zehnder Modulator (MZM) . Such a modulator is well-known in the art from for example US20080094123.

In particular, the invention relates to the stabilisation of the operation point of an MZM which constitutes a part of an integrated optical transmitter, such as an optical transmit^¬ ter for high frequency data- and/or telecommunication. Such a transmitter typically comprises a semiconductor laser and a semiconductor MZM. The transmission performance of such a transmitter is determined among other things by the output power of the laser light entering the modulator and the crossing point (CR) and extinction ratio (ER) of the mod^¬ ulated optical signal that is output from the modulator. Over time, the properties of semiconductor components such as a laser and an MZM may drift due to ageing, thermal varia^¬ tions and so on. In order to achieve a reliable output from a transmitter, there is therefore a need to stabilise the com^¬ ponents. One possibility is to stabilise both a semiconductor laser and a semiconductor MZM using feedback loops that measure an output power of the respective component and then to vary an input parameter so as to control and stabilise the output signal. The measured output power of the MZM will depend on the light input power into the MZM; the optical losses and absorption; and the degree of constructive interference between the two beams at the output coupler of the MZM, in other words the phase difference between its two arms. Thus, the stabilisa^¬ tion of a transmitter that utilizes a laser and a semiconduc^¬ tor MZM will only work well as long as the laser light input into the MZM is stable.

In some approaches, the output waveform of a transmitter is stabilised by disposing a pair of photo-detectors after an MZM. EP0859262 discloses a locking method based on the detec^¬ tion of the interference pattern between radiated light and signal light at the output of the MZM. The measured value is used to stabilise the operation point of the MZM by utilizing a feedback loop. A problem with this approach is that there are additional space requirements and costs associated with the two additional detectors and the accurate placement of these within the transmitter assembly.

In case the transmitter is in the form of an integrated circuit, it is often difficult to measure the light output from the laser into the MZM. The reason for this is that it is complicated and/or expensive to arrange an additional wave^¬ guide on the integrated chip for tapping off part of the light output from the laser. Also, an extra photodetector would be necessary for detecting the tapped off light. This problem is particularly important in the very compact types of integrated photonic circuits called monolithically inte^¬ grated and hybrid integrated. In order to mitigate this prob^¬ lem without adding too much costly complexity to the trans^¬ mitter, it has been proposed that the laser power be measured indirectly by measuring the output power of the MZM. Evident- ly, this strategy will however lead to other problems since it is not possible to distinguish between laser drift and MZM drift . Moreover, there is a problem that the optical coupling effi^¬ ciency between the laser and the MZM can vary over time. Such variation also affects the output power of the MZM. The present invention solves the above problems.

Thus, the invention relates to a method for controlling the operation point of a semiconductor Mach Zehnder Modulator (MZM) , the MZM comprising an input waveguide which is divided into two intermediate waveguides and then joined together in an output waveguide, in which method the input power from a laser into the MZM is stabilised and the operation point of the MZM is then controlled given the stabilised input from the laser, and is characterised in that the laser and the MZM are caused to be monolithically integrated or hybrid inte^¬ grated, in that the photocurrent in at least one of said input- and/or intermediate waveguides of the MZM is measured and in that the measured value is fed to a control circuit, which control circuit is caused to stabilise the input light power fed into the MZM based upon the measured photocurrent value .

The invention also relates to an integrated transmitter ac^¬ cording to claim 12.

In the following, the invention will be described in detail, with reference to the appended drawings, in which:

Figure 1 is a principal diagram illustrating a typical eye diagram of the modulated optical output signal of an MZM;

Figure 2 is a circuit diagram over a first preferred embodi^¬ ment of the present invention; and Figure 3 is a circuit diagram over a second preferred embodi^¬ ment of the present invention.

Figure 1 shows graphically a typical eye diagram of the opti- cal output signal of an MZM when modulated with a non-return to zero digital data pattern. The output power is modulated between a lower output power PO and a higher output power PI . Some of the most relevant parameters characterising the eye diagram are the average power P_av; the ratio P1/P0, which is called extinction ratio (ER) ; and the crossing point (CRS) , which indicates where the rising slope crosses the falling slope. The crossing point is determined by the duty-cycle of the signal and is quantified such that a 50% duty-cycle rend^¬ ers 50% crossing point. These parameters in turn determine the transmission performance of the MZM, and are collectively referred to as the "operation point" of the MZM herein. Simi^¬ lar parameters and operation points can be defined for the optical signal of a MZM when modulated with a return to zero digital pattern.

Figure 2 illustrates a first embodiment of the invention. Laser light emitted by a laser 1 enters an MZM 2 comprising an input waveguide 2a, two intermediate waveguides 2b, 2c and two output waveguides 2d, 2e. The light is split by an opti- cal divider 2f into two beams travelling along the two respective intermediate waveguides 2b, 2c, which thereafter meet within an optical combiner 2g, in turn terminating in the output waveguides 2d, 2e. The laser 1 and the MZM 2 are of semiconductor type and inte^¬ grated in the same capsule. Preferably, they are monolithi- cally integrated or hybrid integrated. Herein, the term "monolithically integrated" means that the laser 1 and the MZM 2 are made from a single monolithical semiconductor structure. Moreover, the term "hybrid integrated" means that the laser 1 and the MZM 2 are realised as individual semiconductor components that are closely assem^¬ bled either on the same substrate or on separate substrates but contained within a single capsule.

The MZM arms 2b and 2c typically consist of a semiconductor optical waveguide structure that is grown epitaxially on a semiconductor substrate of a first conductivity type, and clad by one or more semiconductor layers of a second conduc^¬ tivity type. The waveguide structure typically consists of at least one epitaxial layer having a refractive index that is larger than the refractive index of the cladding material above and below that layer, and where the lateral width of the waveguide is defined by etching and then cladding the etched surfaces with one or more materials having a refrac^¬ tive index that is smaller than the active region layer (s), such as a semiconductor material, dielectrics, and/or air. By the application of a voltage difference to electrical contacts that are placed on the top cladding material and on the substrate, an electrical field of reversed polarity is generated across a portion of the waveguide structure, this portion being referred to as the waveguide active region. In response to an electric field of reverse polarity and of increasing magnitude, the refractive index of the active region will typically increase, and the optical absorption of the active region will also typically increase. The voltage- induced change in the refractive index and the absorption in one or both arms of the MZM is utilized to adjust the optical interference (constructive or destructive) of the two light beams in the combiner 2g, and thereby to modulate the ampli- tude and/or phase of the light signal that is emitted from the MZM. The partial absorption of light in each MZ arm 2b and 2c due to the DC voltage and the applied modulation vol^¬ tage will result in a photocurrent in the electrical circuit which is used to apply a voltage across each MZ arm. The magnitude of the photocurrent in each MZ arm will typically increase monotonically with increasing optical beam intensi^¬ ty, so that a lookup table can be used to relate the ampli^¬ tude of the detected photocurrent to the amplitude of the optical power that is input into that MZ arm, as a function of the applied voltages on that arm and of the wavelength of the light beam.

The output signal from the MZM 2 is modulated using one or more high-frequency (radio frequency) voltages applied to one or both of the intermediate waveguides 2b, 2c, modulating the light travelling through the waveguide in question by modifying the refractive index of the waveguide material. It is preferred that the light is modulated by two high-frequency voltages, preferably conjugated, that are generated in re^¬ spective radio-frequency generators 7, 8 and applied to re^¬ spective electrodes 5, 6 attached to one respective interme^¬ diate waveguide 2b, 2c each. In order to achieve proper transmission performance, a respective bias voltage is applied by respective voltage sources 101, 201 to each electrode 5, 6. Proper impedance matching is achieved using two respective resistances 102, 202 arranged between each respective voltage source 101, 201 and electrode 5, 6. Decoupling capacitors 104, 204 are also connected to the circuit as shown in the figures. A modulation control electrode 3 is used to control the phase difference between the two intermediate waveguides 2b, 2c. A current or voltage control means 13 is arranged to apply a current or voltage to the modulation electrode 3, whereby the output power of the MZM 2 is modulated by adjusting the phase difference between the intermediate waveguides 2b, 2c and thereby affecting the output power in the output waveguides 2d, 2e of the MZM 2 via constructive or destructive interfe^¬ rence at the optical combiner 2g.

The currents and/or voltages applied by voltage sources 101, 201 and control means 13 affect both the absorption and the phase of the light travelling through the intermediate wave^¬ guides 2b, 2c, and are selected during manufacturing in order to obtain adequate transmission performance in terms of the operation point as defined by P_av, ER and CRS .

According to the present invention, the input light power from the laser 1 into the MZM 2 is first stabilised, and the output power of the MZM 2 is then controlled given the stabi^¬ lised input from the laser 1. By first stabilising the input power from the laser 1 so that there are no fluctuations in the light input into the MZM 2 disturbing the output power of the MZM 2, a simple but accurate control mechanism for con- trolling the MZM 2 output power can be achieved, as is de^¬ scribed in the following.

According to a preferred embodiment, a minor part of the optical output power from MZM 2 is tapped from one of the output waveguides 2d, 2e of the MZM using a splitter 11, and is sensed using a detector 12 which converts the optical signal into a signal which is readable by the control means 13. The detector 12 may be a conventional photodetector . Alternatively, a monolithically integrated detector (not shown) , arranged at one or both of the output arms 2d, 2e of the MZM 2, may be used to tap off a minor part of the optical output power from the MZM 2. According to a preferred embodi- ment, such a monolithically integrated detector is arranged to measure the optical power indirectly by measuring the photocurrent in the output waveguide 2d and/or 2e using a monolithically integrated electrode, in a way similar to the one described below in connection to Figure 3 and electrode 306. The control means 13 in turn controls the current or voltage applied to the electrode 3 on the waveguide 2b of the MZM 2 as described above. This way, a feedback control me^¬ chanism is established which can achieve a stable, predeter^¬ mined output power of the MZM 2. Given that the light input into the MZM 2 from the laser 1 is stabilised, such feedback control can be highly efficient.

It is preferred that the modulation control electrode 3 is a separate electrode, as illustrated in figure 2, located along one of the intermediate waveguides 2b, 2c of the MZM 2 and used to control the phase difference between the two interme^¬ diate waveguides 2b, 2c.

According to the present invention, the photocurrent in at least one of the input 2a and/or intermediate 2b, 2c wave^¬ guides of the MZM 2 is measured by a measuring means, and the measured value is fed to a control circuit 4, which control circuit 4 is arranged to stabilise the input light power fed into the MZM 2 from the laser 1. Since the variation in the photocurrent in the waveguides 2a-2c of the MZM 2 as a func^¬ tion of the amount of light travelling through it is known, the measuring means and the control circuit 4 together form a feedback control system which is capable of automatically stabilising the light input from the laser 1.

According to one preferred embodiment, the control circuit 4 comprises a power control circuit which is arranged to stabi^¬ lise the laser 1 output light power. In other words, the power control circuit controls the input light power fed into the MZM 2 by adjusting the output power from the laser 1. The control is based upon the measured photocurrent value.

According to another preferred embodiment, the control cir^¬ cuit 4 comprises a coupling control circuit arranged to sta^¬ bilise the optical coupling between the laser 1 and the MZM 2. In other words, the light input into the MZM 2 is con- trolled by adjusting the coupling efficiency between the laser 1 and the MZM 2. In this case, the control is also based upon the measured photocurrent value. See below for a more detailed discussion regarding such control. It is to be understood that these two methods of controlling the light input into the MZM 2 may be used separately or in combination with each other, depending on the actual application. According to a preferred embodiment, the measuring means comprises a resistance 102, 202, the terminals of which are connected to the respective input ports of an operational amplifier 105, 205. The resistance 102, 202 is connected in series between the respective voltage source 101, 201 and the respective electrode 5, 6, and hence the voltage drop over the resistance 102, 202, which is picked up by the operation^¬ al amplifier 105, 205, is a direct measure of the photocur^¬ rent in the respective waveguide 2b, 2c. According to a very preferred embodiment, the photocurrent is measured by means of an existing electrode 5, 6 for bias voltage at a waveguide 2b, 2c of the MZM 2. This case is illustrated in figure 2, in which the already existing elec- trodes 5 and 6, normally used to apply the bias voltage to the respective waveguide 2b, 2c, are also used to measure the photocurrent through the waveguide 2b, 2c. By using an al^¬ ready existing electrode, a very cost-efficient stabilisation can be achieved.

The photocurrent may be measured in only one intermediate waveguide. This will result in a simple solution. However, it is preferred that the photocurrent is measured in both inter^¬ mediate waveguides 2b, 2c as is illustrated in figure 2, and that both measurement values are fed to the control circuit 4.

It has been found that several important characteristics of the present transmitter can be controlled and stabilised based on the measured photocurrent from each of the two phase modulation arms of an MZM. As indicated above, such control and stabilisation may be carried out by stabilising the laser 1 output power and/or the optical coupling between the laser 1 and the MZM 2.

Conventional lookup tables can be used to relate the magni^¬ tude of the changes in the two measured photocurrents to the required change in the operating conditions of the laser 1, the operating conditions of the MZM 2, and/or the operating conditions of any present, actively, that is electrically, thermally or the like, controlled optical coupling systems (not shown in the figures) between the laser 1 and MZM 2. Such operating conditions may then be altered in order to maintain one or more transmitter characteristics, such as the transmitter output power, at a predetermined value.

It is to be understood that the output power of the laser 1 can be adjusted by the power control circuit in different ways. For example, the power control circuit can alter the gain-providing-current in the laser 1 directly or the gain- providing-current into an amplifier (not shown) arranged in between the laser 1 and the MZM 2, or the attenuation of a variable attenuator (not shown) arranged in between the laser 1 and the MZM 2, depending on the optical circuit design.

In order to control the optical coupling between the laser 1 and the MZM 2, a slight positional or angular change may be imparted to the coupling of the optical laser beam into the MZM 2. In case the transmitter is hybrid integrated, the coupling may for example be via hybrid integrated microme- chanical (MEMS) mirrors, the orientation of which may be adjusted to affect the coupling. In case the transmitter is monolithically or hybrid integrated, the coupling may for example be based on multimode interference (MMI) or Arrayed Waveguide Grating (AWG) that may be controlled so as to af^¬ fect the coupling. The simultaneous detection of the photocurrent in both inter^¬ mediate waveguides of the MZM can thus be used to draw con^¬ clusions regarding the type of compensation, in terms of correcting the optical coupling between the laser array and the MZM, that needs to be made in order to keep the output power and the operation point of the MZM at a predetermined value . According to a first preferred embodiment, a simultaneous decrease in the measured photocurrent in both intermediate waveguides 2b, 2c, when there has been no change in the MZM 2 operational biases or temperature, is interpreted as a reduc- tion in either the optical output power of the laser 1 and/or a reduction in the coupling efficiency between the laser 1 output beam and the input waveguide 2a. Therefore, as a re^¬ sponse to such decrease it is preferred to either increase the output power of the laser 1 or to attend to any arisen coupling problems between the laser 1 and the MZM 2.

Similarly, a simultaneous increase in the photocurrent in both waveguides 2b, 2c, when there has been no change in the MZM 2 operational biases or temperature, is interpreted as either an increase in the optical output power of the laser 1 and/or in the coupling efficiency between the laser 1 output beam and the input waveguide 2a. The preferred response is therefore to either lower the output power of the laser 1 or to attend to any arisen coupling problems in a similar way.

According to a second preferred embodiment, a simultaneous change in the measured photocurrents in the intermediate waveguides 2b, 2c with opposite signs is interpreted as a change in the input optical power splitting ratio of the MZM 2. Such change could be caused for example by a slight posi^¬ tional or angular change in the coupling of the optical laser beam into the MZM 2.

The input splitting ratio of an MZM usually affects both the ER and the chirp of the output optical waveform from the MZM. Thus, it is preferred that a conventional lookup table is used to adjust operational settings or temperature of either the MZM 2 or laser 1, or to adjust the optical coupling be- tween the laser 1 and MZM 2, in order to maintain one or more transmitter characteristics, such as the transmitter output power, dynamic ER, dynamic chirp, or optical eye CRS, at a predetermined value.

According to a third preferred embodiment, a change in the measured photocurrent in only one of the intermediate wave^¬ guides 2b, 2c, when there is no substantial change in the measured photocurrent of the other intermediate waveguide, is interpreted as degradation in the properties of the interme^¬ diate waveguide in which the measured photocurrent has changed. The degradation can, for instance, be due to ageing or to a change in the electrical leakage rate to that wave^¬ guide from one or more components arranged within proximity of the waveguide. In this case, it is preferred that an alarm is set off in case the degradation exceeds a specified prede^¬ termined threshold, so that the transmitter can be removed from the transmission network. According to a very preferred embodiment, the output power of the laser 1, which is controlled based upon a change in the measured photocurrent, is only updated when the measured change in photocurrent at a given point in time has the same sign in both waveguides 2b, 2c. In other words, only a de- tected change, when both measured photocurrents either in^¬ crease or decrease at the same time, will be used to alter the output power of the laser 1. Under many operating conditions, it is very probable that such simultaneous changes in the measured photocurrents are due to fluctuations in the output power of the laser 1. Therefore, the feedback stabili^¬ sation of the laser 1 will be reliable. Figure 3 shows a second preferred embodiment. Same reference numerals denote corresponding parts in figures 2 and 3.

In figure 3, the photocurrent is measured in the input wave^¬ guide 2a of the MZM 2 in a way similar to the one described above in connection to figure 2, using a measurement resis^¬ tance 302, an operational amplifier 305 and a voltage source 301 for applying a bias voltage over a power detecting electrode 306 arranged at the input waveguide 2a. This way, a more accurate reading of the laser 1 output power may be achieved in certain applications.

According to an alternative embodiment, the photocurrent is measured in a corresponding manner, however using an electrode arranged on the optical divider 2f.

According to yet another, alternative embodiment, the photo^¬ current is measured in a corresponding manner, however using an electrode arranged across both intermediate waveguides 2b, 2c at a position in immediate connection and adjacent to the optical divider 2f.

The present inventors have found that the operation point of an MZM in a monolithically or hybrid integrated circuit can efficiently and cheaply be kept unaltered over time if the input power to the MZM is kept constant, using the measure^¬ ment of the photocurrent in one or several MZM waveguides to control the light power from the laser into the MZM, and if the output power of the MZM is measured and used in a feed^¬ back loop to control the phase difference in the MZM.

This is particularly true in the case where CRS is in the vicinity of 50%. Namely, if the laser input light level is held constant and CRS is in the vicinity of 50%, it can be shown that it is possible to maintain the most relevant eye diagram parameters (P_aV/ PI_/ P0, and CRS) constant, and there^¬ by maintaining the transmission performance unaltered, by applying a servo loop that stabilises the output power of the MZM by controlling the phase difference between the two MZM arms as described above.

It is preferred that each electrode 3, 5, 6, 306 described above is monolithically integrated with the MZM 2.

Above, preferred embodiments have been described. However, the invention may be modified within the scope of the ap^¬ pended claims.

For example, the photocurrent may be measured in both the intermediate waveguides of an MZM and at the same time in the input waveguide of the MZM. Moreover, several MZM:s, arranged in an array or otherwise, may be fed by one and the same laser, in which case the out^¬ put power of the laser may be controlled based upon measured photocurrents in one or several of the MZM:s. Furthermore, the MZM may be integrated with an array of las^¬ ers, where a specific laser of the array of lasers is coupled into the MZM. In case the transmitter is hybrid integrated, the coupling may for example be via hybrid integrated micro- mechanical (MEMS) mirrors. In case the transmitter is mono- lithically or hybrid integrated, the coupling may for example be based on multimode interference (MMI) or Arrayed Waveguide Grating (AWG) . In the case with a laser array, as in the case in which a single laser is integrated with the MZM as de- scribed above, the simultaneous detection of the photocurrent in both intermediate waveguides of the MZM can be used to draw conclusions regarding the type of compensation that needs to be made in order to keep the output power and the operation point of the MZM at a predetermined value, notably for correcting the coupling between the laser array and the MZM.

Regarding the feedback mechanism for controlling the output power of the MZM, instead of applying a current or a voltage to a separate modulation control electrode 3, the current or voltage control means 13 may adjust the phase difference between the two intermediate waveguides 2b, 2c by applying a current or voltage directly to one or both of the electrodes 5, 6 of the intermediate waveguides 2b, 2c. This is especial^¬ ly preferred in case the laser 1 is stabilised in the way which is described in connection to Figure 3 above.

Thus, the invention is not limited to the above-described embodiments.

Claims

C L A I M S

1. Method for controlling the operation point of a semiconductor Mach Zehnder Modulator (MZM) (2), the MZM (2) compris- ing an input waveguide (2a) which is divided into two inter^¬ mediate waveguides (2b, 2c) and then joined together in an output waveguide (2d,2e), in which method the input power from a laser (1) into the MZM (2) is stabilised and the oper^¬ ation point of the MZM (2) is then controlled given the sta- bilised input from the laser (1) , c h a r a c t e r i s e d i n that the laser (1) and the MZM (2) are caused to be monolithically integrated or hybrid integrated, in that the photocurrent in at least one of said input- and/or interme^¬ diate waveguides (2a, 2b, 2c) of the MZM (2) is measured and in that the measured value is fed to a control circuit (4), which control circuit (4) is caused to stabilise the input light power fed into the MZM (2) based upon the measured photocurrent value.

2. Method according to claim 1, c h a r a c t e r i s e d i n that the control circuit (4) is caused to comprise a power control circuit which is caused to be arranged to sta^¬ bilise the laser (1) output light power, and hence the input light power fed into the MZM (2), based upon the measured photocurrent value.

3. Method according to claim 1 or 2, c h a r a c t e r i s e d i n that the photocurrent is measured in one of the intermediate waveguides (2b, 2c) of the MZM (2) .

4. Method according to claim 1 or 2, c h a r a c t e r i s e d i n that said photocurrent is measured in the input waveguide (2a) or the optical divider (2f) of the MZM (2), alternatively using an electrode arranged across both intermediate waveguides (2b, 2c) at a position in immediate connection and adjacent to the optical divider (2f) .

5. Method according to claim 1 or 2, c h a r a c t e r i s e d i n that said photocurrent is measured in both intermediate waveguides (2b, 2c) and in that the output power of the laser (1), which is controlled based upon a change in the measured photocurrent, is only updated when the measured change in the photocurrent has the same sign in both wave^¬ guides (2b, 2c) .

6. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the control circuit (4) is caused to comprise a coupling control circuit which is caused to be arranged to stabilize the optical coupling between the laser (1) and the MZM (2), and hence the input light power fed into the MZM (2), based upon the measured photocurrent value .

7. Method according to claim 6, c h a r a c t e r i s e d i n that said photocurrent is measured in both intermediate waveguides (2b, 2c), and in that the coupling control circuit is caused to be arranged to impart a positional or angular change to the coupling of the optical laser beam into the MZM (2) based upon a simultaneous increase or decrease in both the measured photocurrents or upon a simultaneous increase in one of the measured photocurrents and a decrease in the oth^¬ er .

8. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that said photocurrent is meas^¬ ured by means of an existing electrode (5, 6) for bias current at an input or intermediate waveguide (2a, 2b, 2c) of the MZM (2) .

9. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the optical output power from said MZM (2) is sensed, in that the sensed output power is used to control the current or voltage applied to a con^¬ trol electrode arranged at one of the arms of the MZM (2) and in that the current or voltage applied to said electrode is adjusted such as to obtain a predetermined output power of the MZM (2) .

10. Method according to claim 9, c h a r a c t e r i s e d i n that the control electrode is caused to be a separate electrode (3) arranged along one of the intermediate wave^¬ guides (2b, 2c) in the MZM (2) used to control the phase dif^¬ ference between the two intermediate waveguides (2b, 2c).

11. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the laser (1) and the MZM

(2) are caused to be monolithically integrated.

12. Integrated transmitter comprising a semiconductor Mach Zehnder Modulator (MZM) (2) and a laser (1) arranged to feed the MZM (2) with light, the MZM (2) comprising an input waveguide (2a) which is divided into two intermediate waveguides (2b, 2c) and then joined together in an output waveguide (2d,2e) , c h a r a c t e r i s e d i n that the laser (1) and the MZM (2) are monolithically integrated or hybrid inte- grated, in that a measurement means ( 101 , 102 , 105 ; 201 , 202 , 205 ; 301,302,305) is arranged to measure the photocurrent in at least one of said input- and/or intermediate waveguides (2a, 2b, 2c) of the MZM (2) and to feed the measured value to a control circuit (4), which control circuit (4) is in turn arranged to stabilize the input light power fed into the MZM (2) based upon the measured photocurrent value.

13. Integrated transmitter according to claim 12, c h a - r a c t e r i s e d i n that the control circuit (4) com^¬ prises a power control circuit which is arranged to stabilize the laser (1) output light power, and hence the input light power fed into the MZM (2), based upon the measured photocur^¬ rent value.

14. Integrated transmitter according to claim 12 or 13, c h a r a c t e r i s e d i n that the measurement means (101, 102, 105; 201, 202, 205) is arranged to measure the photo^¬ current in one of the intermediate waveguides (2b, 2c) of the MZM (2) .

15. Integrated transmitter according to claim 12 or 13, c h a r a c t e r i s e d i n that the measurement means (301,302,305) is arranged to measure the photocurrent in the input waveguide (2a) or the optical divider (2f) of the MZM (2), alternatively using an electrode arranged across both intermediate waveguides (2b, 2c) at a position in immediate connection and adjacent to the optical divider (2f) .

16. Integrated transmitter according to claim 12 or 13, c h a r a c t e r i s e d i n that the measurement means (101, 102, 105; 201, 202, 205) is arranged to measure the photo^¬ current in both intermediate waveguides (2b, 2c) and in that the power control circuit 4 is arranged to only alter the output power of the laser (1) when the measured change in photocurrent has the same sign in both waveguides (2b, 2c) .

17. Integrated transmitter according to any one of the claims 12-16, c h a r a c t e r i s e d i n that the control circuit (4) comprises a coupling control circuit which is arranged to stabilize the optical coupling between the laser (1) and the MZM (2), and hence the input light power fed into the MZM (2), based upon the measured photocurrent value.

18. Integrated transmitter according to claim 17, c h a r a c t e r i s e d i n that the measurement means (101, 102, 105; 201, 202, 205) is arranged to measure the photo^¬ current in both intermediate waveguides (2b, 2c), and in that the coupling control circuit is arranged to impart a posi^¬ tional or angular change to the coupling of the optical laser beam into the MZM (2) based upon a simultaneous increase or decrease in both the measured photocurrents or upon a simul^¬ taneous increase in one of the measured photocurrents and a decrease in the other.

19. Integrated transmitter according to any one of the claims 12-18, c h a r a c t e r i s e d i n that the measurement means (101, 102, 105; 201, 202, 205; 301, 302, 305) is arranged to measure said photocurrent by means of an existing electrode (5, 6) for bias current at an input or intermediate waveguide (2a, 2b, 2c) of the MZM (2).

20. Integrated transmitter according to any one of the claims 12-19, c h a r a c t e r i s e d i n that a sensor (12) is arranged to sense the optical output power from said MZM (2), in that a current or voltage control means (13) is arranged to adjust the current or voltage applied to a control elec^¬ trode arranged at one of the arms of the MZM (2), so that the sensed output power is used to control the current or voltage applied to the control electrode such as to obtain a prede^¬ termined output power of the MZM (2) .

21. Integrated transmitter according to claim 20, c h a r a c t e r i s e d i n that the control electrode is a separate electrode (3) arranged along one of the intermediate waveguides (2b, 2c) in the MZM (2) used to control the phase difference between the two intermediate waveguides (2b, 2c) .

22. Integrated transmitter according to any one of the claims 12-21, c h a r a c t e r i s e d i n that the laser (1) and the MZM (2) are monolithically integrated.