WO2019068334A1

WO2019068334A1 - Wavelength monitoring and/or controlling device, laser system with such a device and method for operating such device

Info

Publication number: WO2019068334A1
Application number: PCT/EP2017/075419
Authority: WO
Inventors: Alberto Rampulla; Paolo Zago
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2017-10-05
Filing date: 2017-10-05
Publication date: 2019-04-11
Also published as: CN111194528B; CN111194528A

Abstract

The present invention provides a wavelength monitoring and/or controlling device, preferably a wavelength locker, for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a tunable laser, the device comprising a first beam splitter configured to divide the original laser beam into two beams with a predetermined angular displacement, and configured to differently polarize the two beams, such that a first beam of the two beams has a first polarization and a second beam of the two beams has a second polarization; and an etalon filter configured to filter the two polarized beams with the predetermined angular displacement. The present invention further provides a laser system comprising at least one such wavelength monitoring and/or controlling device according to the present invention and a method for operating such wavelength monitoring and/or controlling device according to the present invention.

Description

WAVELENGTH MONITORING AND/OR CONTROLLING DEVICE, LASER SYSTEM WITH SUCH A DEVICE AND METHOD FOR OPERATING SUCH

DEVICE

TECHNICAL FIELD

The present invention relates to a wavelength monitoring and/or controlling device, preferably a wavelength locker, for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a tunable laser; a laser system comprising at least one such wavelength monitoring and/or controlling device according to the present invention and a method for operating such wavelength monitoring and/or controlling device according to the present invention.

BACKGROUND

In dense wavelength division multiplexing (DWDM) optical systems, laser sources are frequency-tuned to several channels which are optically modulated and multiplexed into a single mode fiber for long haul communications link. An example of a DWDM optical system, in which laser sources are frequency-tuned to several channels which are optically modulated and multiplexed into a single mode fiber, is schematically shown in Fig. 1. Wavelength lockers (WL) are key components used to ensure stable and accurate wavelength respectively frequency monitoring and laser control, providing an electrical feedback proportional to the frequency deviation from the desired value (f TARGET). This feedback may be used for laser active control in a closed loop scheme to compensate the deviation (see Fig. l).

A typical wavelength locker arrangement consists of a Fabry-Perot interferometric filter, also known as etalon filter, beam splitters (BS) and photodiodes (PD). An example of such a wavelength locker used in DWDM optical systems is schematically shown in Fig. 2. The unfiltered input beam power is split by a beam splitter into two beams travelling along different optic paths. In a first optic path the unfiltered input beam power is measured by a monitor photodiode (PD monitor) and in a second optic path the optical beam is filtered by an etalon filter and detected by a signal photodiode (PD signal). The ratio between the measurement result of the monitor photodiode and the measurement result of the signal photodiode (PD signal to PD monitor ratio) is the electrical output signal which is related to the deviation of the actual frequency of the input beam from the desired frequency.

The etalon filter response shows equally spaced transmission peaks because of the interferometric nature of the filter, with position and spacing (free spectral range, FSR) of the peaks depending on the refractive index, input beam angle of incidence (AOI) and thickness of the bulk material of the etalon filter. If the transmission peaks match the system channels frequency grid of the DWDM optical system, the wavelength locker can act as a frequency reference in the transmission link. This is particularly suitable for systems with channels equally spaced at fixed frequencies, such as standard ITU 50GHz or 100GHz grid. Fig. 3 schematically shows an example of such a transmission peak pattern generated by an etalon filter for a laser beam, wherein the transmission peaks match the system channels frequency grid of a DWDM optical system. In Fig. 3 the channels are marked as CHANNEL i-1, CHANNEL i and CHANNEL i+1. Further, a frequency deviation between the actual frequency of a laser beam and the CHANNEL i is exemplarily shown in Fig. 3.

However continuous frequency tuning of the laser source may be required for new optical communication systems whose channels are not limited to a fixed grid but can set to any value over a broad tuning range (e.g. 1525 nm to 1575 nm). In these applications standard wavelength lockers are unable to detect frequency deviation with respect to target values located near the peak or midway between peaks of a transmission peak pattern generated by an etalon filter, where the output signal has no change against frequency variation (zero derivative).

The existing solutions which address the continuous tuning issue may be grouped into sets with similar basic principle: According to a first principle thermal tuning of the etalon filter by some cooler/heater element may be performed, which achieves filter peaks shift by refractive index change with temperature. Thus, according to some factory calibration, for any target channel frequency the filter temperature is adjusted so that the filter response can be used as frequency deviation feedback. According to a second principle a double etalon assembly with fixed transmission peaks conveniently offset to have a nonzero derivative over the continuous range may be used. In such a case the input beam is split between two filters with shifted response one to the other, wherein the combination of the filtered signals overcomes the limitations of the single filter feedback.

According to a third principle wavelength lockers integrating etalon made of some birefringent material are used, so that orthogonal polarizations have slightly different transmission peaks. By controlling the input beam polarization a similar response to the double etalon scheme can be achieved to ensure frequency deviation feedback over continuous range.

The major drawback of the first principle is an increase of the device power consumption, as thermal tuning requires some extra power for heating/cooling compared to an a thermal or fixed temperature approach. The second principle needs doubling components and thus doubles the costs. Moreover the increased parts count leads to a larger device size, making the integration rather challenging in particular with co-packaged laser arrangements. The third principle may be compact and power saving, but custom etalons of birefringent material are not cheaply available in the market. In order to work properly, a very well controlled filter response is required for both polarization states, which causes problems in manufacturing and raises the costs.

SUMMARY

In view of the above-mentioned problems and disadvantages, the present invention aims to provide a continuous frequency tuning of a laser source, in particular for DWDM optical systems, without the above mentioned drawbacks and disadvantages of the state of the art solutions which address the continuous tuning issue. The present invention has thereby the object to provide a wavelength monitoring and/or controlling device, in particular a wavelength locker that enables continuous frequency tuning of the laser source, in particular for DWDM optical systems, without the above mentioned drawbacks and disadvantages. The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims. In particular the present invention proposes a wavelength monitoring and/or controlling device, preferably a wavelength locker, a laser system comprising at least one such wavelength monitoring and/or controlling device and a method for operating such a wavelength monitoring and/or controlling device.

A first aspect of the present invention provides a wavelength monitoring and/or controlling device, preferably a wavelength locker, for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a preferably tunable laser, the device comprising

a first beam splitter configured to divide the original laser beam into two beams with a predetermined angular displacement, and configured to differently polarize the two beams, such that a first beam of the two beams has a first polarization and a second beam of the two beams has a second polarization; and

an etalon filter configured to filter the two polarized beams with the predetermined angular displacement.

As a result of the angular displacement between the two beams, the filtering of the two beams by the etalon filter results in two transmission peak patterns, which are shifted with respect to each other. These two transmission peak patterns shifted with respect to each other may be used for the continuous frequency tuning of the tunable laser. Since the shift between the two transmission peak patterns generated by the single etalon filter depends on the angular displacement between the two beams, an exact positioning of the etalon filter in the optic path formed by the first beam splitter and the etalon filter is not necessary.

Therefore, the wavelength monitoring and/or controlling device according to the first aspect of the invention is advantageous, as the etalon filter may be easily installed in the optic path behind the first beam splitter without the need of lengthy and complicated adjustments of the position and/or angular positioning of the etalon filter in the optic path. Namely, as long as the two beams with the predetermined angular displacement generated by the first beam splitter may enter the etalon filter, the angle of installation of the etalon filter in the optic path of the wavelength monitoring and/or controlling device may vary in order to generate two transmission peak patterns shifted with respect to each other.

Thus, the proposed invention provides a wavelength monitoring and/or controlling device, in particular a wavelength locker, which comprises an etalon filter probed with a pair of beams at the same wavelength respectively frequency (which wavelength respectively frequency is the wavelength respectively frequency of the original laser beam received from the tunable laser) travelling through the etalon filter at different angles of incidence, i.e. the first beam travels at a first angle of incidence and the second beam travels at a second angle of incidence. Preferably, values for the angles of incidence range between 0° to 2°. This avoids high filter insertion loss.

Since the angles of incidence of the two beams are slightly different, the frequency response of the etalon filter, i.e. the transmissions peak pattern generated by the etalon filter, for one of the two beams shifts with respect to the frequency response of the etalon filter for the other beam of the two beams, because of the dependency of the frequency response on the angle of incidence.

The m-th transmission peak of a transmission peak pattern generated by the etalon filter for a beam travelling through the etalon filter at an angle of incidence may be defined by the following formula: v_m = m

2nl cos(AOI)

Therein," V_m" corresponds to the m-th transmission peak of a transmission peak pattern generated by the etalon filter; "C" corresponds to the speed of light; "71" corresponds to the refractive index of the etalon filter; "/" corresponds to the thickness of the etalon filter and AOl" corresponds to the angle of incidence of the beam travelling through the etalon filter, for which the transmission peak pattern is generated by the etalon filter.

Preferably, the wavelength monitoring and/or controlling device is a wavelength locker that is configured to ensure a stable and accurate wavelength respectively frequency of the original laser beam output by the tunable laser. In particular, the wavelength monitoring and/or controlling device is configured to monitor and/or control the wavelength or frequency of the laser beam emitted by the tunable laser.

In the present invention the wording "laser beam" and "beam" are used as synonyms, which are to be understood as electromagnetic radiation generated by a laser device. That is, the "laser beam" preferably corresponds to an electromagnetic radiation at a certain wavelength or frequency generated by a laser device comprising the characteristics of laser radiation. The laser beam may correspond to a light beam which has a wavelength or frequency in the visible spectrum.

The angular displacement of the two beams corresponds to the difference between the angle of incidence (AOI) of the first beam with respect to the etalon filter and the angle of incidence of the second beam with respect to the etalon filter. The angle of incidence of the first or second beam with respect to the etalon filter corresponds to the angle at which the first respectively second beam enters the etalon filter with respect to the normal vector on the surface of the etalon filter. That is, a beam perpendicularly entering the etalon filter, i.e. a beam entering the etalon filter in the direction of the normal vector on the surface of the etalon filter, comprises an angle of incidence of 0°. In the present application the passages "angular displacement of two beams", "angular displacement of two beams with respect to each other" and "angular displacement between two beams" are to be understood as synonyms.

Preferably, the etalon filter is a Fabry-Perot interferometer, which is preferably made of a transparent plate with two reflecting surfaces. Alternatively, the etalon filter may be made of two parallel highly reflecting mirrors. The etalon filter is preferably configured to generate transmission peak patterns for a beam as a function of the wavelength or frequency of the beam, wherein the peaks in the transmission spectrum correspond to resonances of the etalon filter.

Preferably, the etalon filter is made of glass. According to the present invention, any known etalon filter respectively Fabry-Perot interferometer may be used for implementing the etalon filter of the wavelength monitoring and/or controlling device of the first aspect of the present invention. Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the etalon filter. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

The wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as common optical elements, such as a first beam splitter configured to split and polarize a laser beam into two differently polarized beams with a predetermined angular displacement and an etalon filter, may be used for implementing the wavelength monitoring and/or controlling device of the first aspect of the present invention.

Preferably, the original laser beam may also be received from a non-tunable laser. The original laser beam may be received from any known laser source. Preferably, the first beam splitter and the etalon filter are arranged in an optic path of the wavelength monitoring and/or controlling device, such that the laser light beam received from the tunable laser travels through the first beam splitter and the etalon filter, wherein the laser beam is split by the first beam splitter into two differently polarized beams with an angular displacement. These two beams travel through the etalon filter with a predetermined angular displacement between each other.

The first beam splitter and the etalon filter preferably form an optic path for the original laser beam received from the tunable laser. In the optic path the etalon filter is preferably arranged behind the first beam splitter with respect to the input of the wavelength monitoring and/or controlling device receiving the laser beam from the tunable laser.

The first beam splitter is configured, such that the laser beam travelling through the first beam splitter is divided into two differently polarized beams with an angular displacement between each other. That is, the first beam splitter is configured such that two beams result out of the original laser beam applied to the first beam splitter, wherein the two beams travel in different directions to the etalon filter and are differently polarized. Since the two beams travel in different directions to the etalon filter, they enter the etalon filter at different angles of incidence.

The etalon filter is configured such that for each of the two beams travelling through the etalon filter a transmission peak pattern is generated in dependence on the angle of incidence of the respective beam. That is, the etalon filter is configured to generate for the two beams different transmission peak patterns, as the two beams have different angles of incidence, at which they enter the etalon filter. The difference in the angles of incidence of the first beam and the angle of incidence of the second beams corresponds to the angular displacement between the two beams.

As mentioned already above, the position and spacing of the peaks of a transmission peak pattern generated by the etalon filter for a beam travelling through the etalon filter depends on the refractive index and thickness of the material of the etalon filter as well as the angle of incidence (AOI) at which the respective beam enters the etalon filter. The spacing in optical frequency or wavelength between two successive peaks of a transmission peak pattern is also referred to as free spectral range (FSR).

The two output beams filtered by the etalon filter may be detected by two detection means, such as photodiodes. Thus, proper combination of the detected signals results in feedback with nonzero derivative over the capture range for a feed-back control of the frequency of the laser beam (received from the tunable laser assigned to the wavelength monitoring and/or controlling device) with respect to a desired respectively target frequency ^TARGET in full range of interest. Therefore, the wavelength monitoring and/or controlling device according to the present invention is suitable for frequency deviation measurement with the target frequency frARGET not limited to any fixed grid. By means of the arrangement of the wavelength monitoring and/or controlling device according to the present invention gridless operation of the wavelength monitoring and/or controlling device may be achieved without the need of doubling the components or using expensive parts. The manufacturing process for the wavelength monitoring and/or controlling device is as simple as for„standard" wavelength lockers with similar overall size. Therefore the advantages offered by the wavelength monitoring and/or controlling device according to the first aspect of the present invention are the following:

The wavelength monitoring and/or controlling device according to the present invention may be based on cheap components available off-the-shelf, like glass etalon filter (with e.g. 50 GHz FSR), the first beam splitter in the form of a birefringent beam splitter (e.g. made of YV04 material), which may be also referred to as small angle polarization splitter, and optionally polarization beam splitter. There is no need of fabrication of unconventional optical parts like a birefringent etalon filter.

Further, the components of the wavelength monitoring and/or controlling device according to the present invention are almost insensitive to assembling tolerances because the key parameter, i.e. the predetermined angular displacement between the two beams output by the first beam splitter, for setting the shift between the transmission peak patterns generated by the etalon filter for the two beams slightly depends on the position of the components, i.e. the position of the first beam splitter and/or the etalon filter, in the optic path of the wavelength monitoring and/or controlling device.

Moreover, the wavelength monitoring and/or controlling device may be compact enough to be co-packaged with a laser source, such as a tunable laser, in the same gold box.

In an implementation form of the first aspect, the etalon filter is preferably configured to generate for each beam a transmission peak pattern, wherein, depending on the predetermined angular displacement, the transmission peak pattern generated for the first beam is shifted with respect to the transmission peak pattern generated for the second beam.

With other words, the etalon filter is preferably configured to generate for the first beam entering the etalon filter at a first angle of incidence a first transmission peak pattern and to generate for the second beam entering the etalon filter at a second angle of incidence a second transmission peak pattern, which is shifted with respect to the first transmission peak pattern. The shift depends on the difference between the second angle of incidence and the first angle of incidence, which difference corresponds to the angular displacement between the first and second beam. In a further implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably further comprises two photodiodes configured to detect the two beams filtered by the etalon filter. That is, the wavelength monitoring and/or controlling device according to the present invention preferably comprises two photodiodes, wherein a first photodiode of the two photodiodes is preferably configured to detect respectively measure the transmission peak pattern resulting from the etalon filter filtering the first beam; and a second photodiode of the two photodiodes is preferably configured to detect respectively measure the transmission peak pattern resulting from the etalon filter filtering the second beam. With other words, the first photodiode is preferably configured to detect the transmission peak pattern caused by the first beam travelling through the etalon filter, and the second photodiode is preferably configured to detect the transmission peak pattern caused by the second beam travelling through the etalon filter.

Preferably, any known photodiodes or any known other optical detectors configured to detect a laser beam and the corresponding transmission peak pattern resulting from the filtering of the laser beam by the etalon filter may be used for implementing the two photodiode according to the present invention. That is, the wavelength monitoring and/or controlling device according to the present invention preferably comprises two optical detectors configured to detect the two beams filtered by the etalon filter.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the two photodiodes or optical detectors. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

In another implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably comprises a second beam splitter configured to guide the first beam with the first polarization to a first photodiode of the two photodiodes, and configured to guide the second beam with the second polarization to a second photodiode of the two photodiodes. That is, the wavelength monitoring and/or controlling device according to the present invention preferably comprises a second beam splitter arranged in the optic path of the wavelength monitoring and/or controlling device and/or configured such that the first beam filtered by the etalon filter respectively travelled through and output from the etalon filter is guided to the first photodiode; and such that the second beam filtered by the etalon filter respectively travelled through and output from the etalon filter is guided to the second photodiode. Thus, the second beam splitter is preferably configured to guide the differently polarized first and second beam with the predetermined angular displacement to the first and second photodiode of the two photodiodes, respectively, such that the transmission peak pattern corresponding to the first beam is detected by the first photodiode and the transmission peak pattern corresponding to the second beam is detected by the second photodiode.

Preferably, the second beam splitter is arranged in the optic path of the wavelength monitoring and/or controlling device behind the etalon filter with respect to the input of the wavelength monitoring and/or controlling device receiving the original laser beam from the tunable laser source. That is, starting from the input of the wavelength monitoring and/or controlling device, in the optic path the first beam splitter is followed by the etalon filter, which is preferably followed by the second beam splitter. Thus, the original laser beam received from the external tunable laser is divided respectively split and polarized by the first beam splitter into two differently polarized beams with a predetermined angular displacement between each other.

Next these two differently polarized beams with the predetermined angular displacement are applied to the etalon filter at different angles of incidence. The two differently polarized beams travel through the etalon filter, wherein a transmission peak pattern is generated by the etalon filter for each beam in dependence on the respective angle of incidence. After having travelled through the etalon filter respectively having been filtered by the etalon filter, preferably, a second beam splitter, following the etalon filter in the optic path, guides the two differently polarized beams to different photodiodes. Thus, the first beam of the two beams is guided to a first photodiode and the second beam of the two beams is guided to a second photodiode, wherein the first and second photodiode are configured to detect respectively measure the respective transmission peak pattern resulting from the filtering of the first and second beam by the etalon filter. The second beam splitter may be provided by any known beam splitter that is configured to guide the two beams to different photodiodes respectively optical detectors. Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the second beam splitter. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

In a further implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably comprises two polarization filters arranged at the two photodiodes, respectively, wherein each of the two polarization filters is configured to let only electromagnetic radiation of one polarization type pass through to the respective photodiode.

Preferably, at the first photodiode, to which the filtered first beam is guided by preferably the second beam splitter, a first polarization filter is arranged that is configured to let only electromagnetic radiation of the first beams' polarization type through to the first photodiode. Preferably, at the second photodiode, to which the filtered second beams is guided by preferably the second beam splitter, a second polarization filter is arranged that is configured to let only electromagnetic radiation of the second beams' polarization type through to the second photodiode. This is advantageous, as using the two polarization filters ensures that to each photodiode only electromagnetic radiation of the respective beam of the two differently polarized beams is applied.

According to the invention, any known polarization filters may be used for implementing the optional two polarization filters of the wavelength monitoring and/or controlling device.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the two photodiodes respectively optical detectors. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost- efficient.

In another implementation form of the first aspect, the first beam splitter is preferably a birefringent wedge. A birefringent wedge is a beam splitter that divides a laser beam into two orthogonally polarized beams with a predetermined angular displacement. In particular, the first beam splitter is made of birefringent material having a refractive index depending on the polarization and/or propagation direction of the laser beam. This birefringence causes the phenomenon of double refraction whereby a laser beam, when incident upon the birefringent material of the first beam splitter, is split by polarization into two differently polarized beams taking slightly different optic paths and, thus, having an angular displacement between each other. In the present application the term "birefringent" is synonymous to the term "birefractive".

The first beam splitter is preferably made of birefringent material. The first beam splitter may be made for example of YV04 material.

Preferably, the first beam splitter comprises birefringent material in the form of a crystal.

Preferably, any known beam splitter configured to divide and polarize an original laser beam into two differently polarized, preferably orthogonally polarized, beams with a predetermined angular displacement between each other may be used for implementing the first beam splitter of the wavelength monitoring and/or controlling device according to the present invention.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the first beam splitter. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient. In a further implementation form of the first aspect, the predetermined angular displacement is preferably set by the shape and/or material of the first beam splitter.

That is, preferably, the first angle of incidence of the first beam with the first polarization and the second angle of incidence of the second beam with the second polarization, deviating from the first angle of incidence, are set by the shape and/or material of the first beam splitter. In particular, the travelling direction of the first beam with the first polarization and the travelling direction of the second beam with the second polarization are set set by the shape and/or material of the first beam splitter.

Preferably, the predetermined angular displacement between the two differently polarized beams is such that the two differently polarized beams travel through the etalon filter in the same area, preferably being separated by the second beam splitter at the etalon filter's output.

In another implementation form of the first aspect, the predetermined angular displacement preferably corresponds to an angular displacement between 0° and 2°, more preferably between 0.1° and 1°, most preferred between 0.1° and 0.5°. That is, the difference between the angle of incidence of the first beam and the angle of incidence of the second beam is preferably between 0° and 2°, more preferably between 0.1° and 1°, most preferred between 0.1° and 0.5°.

In a further implementation form of the first aspect, the predetermined angular displacement is preferably such that the transmission peak pattern generated by the etalon filter for the first beam is shifted by 25% of the etalon filter's free spectral range, FSR, with respect to the transmission peak pattern generated by the etalon filter for the second beam.

The free spectral range (FSR) of the etalon filter corresponds to the spacing in optical frequency or wavelength between two successive peaks of the transmission peak pattern generated by the etalon filter for a laser beam applied to the etalon filter. The transmission peak pattern is a function of the wavelength or frequency of the laser beam. The peaks in the transmission spectrum correspond to resonances of the etalon filter. Therefore, the first beam splitter is preferably configured such, that the two differently polarized beams output by the first beam splitter have different travelling directions respectively different angles of incidence for entering the etalon filter. The difference in the travelling directions respectively angles of incidence of the two beams is preferably such that the shift between the transmission peak pattern generated by the etalon filter for the first beam of the two beams and the transmission peak pattern generated by the etalon filter for the second beam of the two beams corresponds to 25% of the etalon filter's free spectral range (FSR) for the frequency respectively wavelength of the original laser beam and, thus, of the two beams being filtered, which originate from the original laser beam.

Preferably, the etalon filter comprises a 30 GHz to 70 GHz FSR, more preferably a 40 GHz to 60 GHz FSR, most preferred a 50 GHz FSR.

Preferably, the predetermined angular displacement is such that the transmission peak pattern generated by the etalon filter for the first beam is shifted by between 10% to 40 %, more preferably 15 % to 30%, most preferred between 23% to 27% of the etalon filter's free spectral range, FSR, with respect to the transmission peak pattern generated by the etalon filter for the second beam. Since the two differently polarized beams output by the first beam splitter have different travelling directions, they travel through the etalon filter at different angles of incidence. The difference in the angles of incidence of the two beams, which is also referred to as angular displacement, causes a shift between the first transmission peak pattern generated by the etalon filter for the first beam of the two beams and the second transmission peak pattern generated by the etalon filter for the second beam of the two beams. That is, the second transmission peak pattern is shifted with respect to the first transmission peak pattern respectively the first transmission peak pattern is shifted with respect to the second transmission peak pattern as a result of the difference between the travelling directions and, thus, between the angles of incidence of the two differently polarized beams travelling through the etalon filter.

In another implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably further comprises a third photodiode configured to monitor the power of the original laser beam, wherein the first beam splitter is configured to reflect a part of the original laser beam, preferably 1% to 2% of the original laser beam, to the third photodiode.

In a further implementation form of the first aspect, the first beam splitter is preferably configured to orthogonally polarize the two beams.

In another implementation form of the first aspect, the first beam splitter is preferably configured such that the original laser beam is divided into the two beams, which pass through the first beam splitter at different phase speeds and thus become differently polarized.

In particular, the original laser beam is a superposition of two orthogonal polarization beams which travel at the same phase speed in the same direction, when the original laser beam is received by the wavelength monitoring and/or controlling device from the tunable laser. When the original laser beam travels through the first beam splitter, which preferably is made of birefringent material, the two orthogonal polarization beams, which form the original laser beam, preferably travel at different phase speeds due to the different orthogonal polarization and birefringent material, so that the two orthogonal polarization beams forming the original laser beam are decoupled and, thus, are identifiable as two separate differently polarized beams with an angular displacement. In particular, at the output surface of the first beam splitter, the difference in the phase speeds of the two separate differently polarized beams, output by the first beam splitter, lead to the angular displacement according to Snell's law between the two separate differently polarized beams.

In a further implementation form of the first aspect, the first beam splitter is preferably configured to divide the original laser beam into the first beam with a fast polarization and the second beam with a slow polarization, wherein for the second beam the material of the first beam splitter has a higher effective refractive index compared to the first beam for which the material of the first beam splitter has a lower effective refractive index.

In particular, the two beams output by the first beam splitter have the same frequency respectively wavelength (corresponding to the frequency respectively wavelength of the original laser beam received by the wavelength monitoring and/or controlling device from the laser source) but different orthogonal polarization states.

Preferably, the first beam splitter is made of birefringent material, which birefringent material preferably has a refractive index almost constant over the application frequency range (e.g. C-band), so that dispersion effects on the angular displacement are negligible.

In another implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably further comprises input means configured to connect an optical fiber to the device for providing the original laser beam from the tunable laser via the optical fiber to the device.

According to the present invention, any known input means configured for coupling or connecting an optical fiber may be used for implementing the input means of the wavelength monitoring and/or controlling device.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the input means. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

In a further implementation form of the first aspect, the optical fiber is preferably a polarization-maintaining optic fiber (PM optic fiber) configured to provide the original laser beam with a predetermined polarization, preferably with a polarization of 45°, or the optical fiber is preferably a single mode optical fiber (SM optic fiber) configured to provide the original laser beam without polarization or with random polarization.

In another implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably further comprises a polarizer arranged between the input means and the first beam splitter, wherein the polarizer is configured to polarize the original laser beam, preferably with a polarization of 45°. Preferably, the polarizer is configured to polarize the original laser beam with a linear polarization of 45°.

According to the present invention, any known polarizer configured to polarize the original laser beam, preferably with a polarization of 45°, may be used for implementing the polarizer of the wavelength monitoring and/or controlling device.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the polarizer. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

In a further implementation form of the first aspect, the wavelength monitoring and/or controlling device according to the present invention preferably further comprises a lens for collimating the original laser beam provided to the input means of the device.

According to the present invention, any known lens configured to collimate the original laser beam may be used for implementing the lens of the wavelength monitoring and/or controlling device.

Thus, the wavelength monitoring and/or controlling device of the first aspect of the present invention is advantageous, as standard optic equipment may be used for implementing the wavelength monitoring and/or controlling device, in particular the lens. This makes the implementation of the wavelength monitoring and/or controlling device of the present invention simple and, thus, cost-efficient.

A second aspect of the present invention provides a laser system, comprising

at least one wavelength monitoring and/or controlling device according to the present invention, as described above,

at least one preferably tunable laser configured to emit the original laser beam, and at least one optical element for guiding at least a part of the original laser beam, preferably 4% to 5% of the original laser beam, from the preferably tunable laser to the wavelength monitoring and/or controlling device. That is, the laser system according to the second aspect of the present invention comprises one or more wavelength monitoring and/or controlling devices according to the present invention; one or more preferably tunable lasers; and one or more optical elements. Preferably, the laser system comprises the same amount of wavelength monitoring and/or controlling device according to the present invention and lasers, wherein each laser is assigned, in particular optically coupled, to a wavelength monitoring and/or controlling device. According to the present invention, any known tunable laser configured to output an original laser beam and tune the frequency respectively wavelength of the laser beam may be used for implementing the one or more lasers of the laser system.

According to the present invention, any known optical element that is configured to guide at least a part of the original laser beam, preferably 4% to 5% of the original laser beam, from the tunable laser to the wavelength monitoring and/or controlling device may be used as the at least one optical element.

In an implementation form of the second aspect, the wavelength monitoring and/or controlling device and the laser source are preferably arranged in a gold box.

In a further implementation form of the second aspect, the laser system according to the present invention preferably further comprises a controller configured to control the wavelength respectively frequency of the original laser beam emitted from the tunable laser on the basis of the two beams generated by the first beam splitter and filtered by the etalon filter of the wavelength monitoring and/or controlling device.

Preferably, the controller is configured to control the wavelength respectively frequency of the original laser beam output by the tunable laser on the basis of the two transmission peak patterns resulting from the filtering of the two beams by the etalon filter of the wavelength monitoring and/or controlling device, wherein the two transmission peak pattern are preferably shifted with respect to each other in dependence on the angular displacement between the two beams.

A third aspect of the present invention provides a method for operating a wavelength monitoring and/or controlling device according to the present invention, as described above, for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a preferably tunable laser, the device comprising a first beam splitter and an etalon filter, wherein

the first beam splitter divides the original laser beam into two beams with a predetermined angular displacement, and differently polarizes the two beams, such that a first beam of the two beams has a first polarization and a second beam of the two beams has a second polarization; and

- the etalon filter filters the two polarized beams with the predetermined angular displacement.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which schematically shows an example of dense wavelength division multiplexing (DWDM) optical system, in which laser sources are frequency-tuned to several channels which are optically modulated and multiplexed into a single mode fiber. schematically shows an example of a wavelength locker used in DWDM optical systems. schematically shows an example of a transmission peak pattern generated by an etalon filter for a laser beam, wherein the transmission peaks match the system channels frequency grid of a DWDM optical system. schematically shows a wavelength monitoring and/or controlling device according to a first aspect of the present invention. schematically shows the angular displacement between two beams originating of an original laser beam with respect to an etalon filter. schematically shows an excerpt of an implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention. schematically shows an example of transmission peak patterns as a result of the etalon filter of the wavelength monitoring and/or controlling device according to the first aspect of the present invention filtering the two beams with the predetermined angular displacement. schematically shows a further implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention. Fig. 9 schematically shows a further implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention. Fig. 10 schematically shows a laser system according to a second aspect of the present invention.

Fig. 11 schematically shows an implementation form of the laser system according to the second aspect of the present invention.

Fig. 12 schematically shows a method according to a third aspect of the present invention.

In the drawings and the following description corresponding elements are referenced by reference signs comprising the same last two digits. In reference signs with three digits, the first digit correspond to the Figure's numbering (Figs. 1 to 9) and the last two digits are used for referencing. In reference signs with four digits, the first two digits correspond to the Figure's numbering (Figs. 10 to 12) and the last two digits are used for referencing. For example, in Fig. 4 the wavelength monitoring and/or controlling device is referenced by the reference sign "401" and in Fig. 10 the wavelength monitoring and/or controlling device is referenced by the reference sign "1001".

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 4 schematically shows a wavelength monitoring and/or controlling device according to a first aspect of the present invention.

The wavelength monitoring and/or controlling device 401 according to Fig. 4 comprises a first beam splitter 403 and an etalon filter 404. Outside the wavelength monitoring and/or controlling device 401 a preferably tunable laser 402 is configured to emit a laser beam B0 (also referred to as original laser beam) with a certain frequency respectively wavelength.

Preferably, the first beam splitter 403 and the etalon filter 404 are arranged in an optic path of the wavelength monitoring and/or controlling device 401, such that the original laser beam BO received from the laser 402 travels through the first beam splitter 403 and the etalon filter 404 arranged behind the first beam splitter 403 in the optic path.

The first beam splitter 403 is configured to divide the original laser beam B0 into two light beams Bl and B2 with a predetermined angular displacement AD, and to differently polarize the two light beams Bl and B2, such that a first light beam Bl of the two light beams has a first polarization and a second light beam B2 of the two light beams has a second polarization. That is, when the original laser beam B0 travels through the first beam splitter 403, the original laser beam B0 is divided into two differently polarized beams Bl and B2 with a predetermined angular displacement AD.

Preferably, the first beam splitter 403 and/or the etalon filter 404 are arranged in the wavelength monitoring and/or controlling device 401 such that the two differently polarized beams B 1 and B2 with the predetermined angular displacement AD output by the first beam splitter 403 are applied to the etalon filter 404, i.e. both reach the surface of the etalon filter 404 and travel through the etalon filter 404. Due to the angular displacement AD between the first beam Bl and the second beam B2, the first beam Bl enters the etalon filter 404 and, thus, travels through the etalon filter 404 at a first angle of incidence, which is different to the second angle of incidence, at which the second beam enters the etalon filter, and, thus, travels through the etalon filter 404 (refer to Fig. 2). The first and second beams B 1 and B2 travel in deviating directions through the optic path of the monitoring and/or controlling device 401 and, thus, enter the etalon filter 404 at different angles of incidence. This difference in the travel directions and angles of incidence of the two beams B 1 and B2 is represented by the angular displacement AD of the two beams Bl and B2.

The etalon filter 404 is configured to filter the two beams B 1 and B2 in order to generate a transmission peak pattern for each of the two beams Bl and B2. Due to the angular displacement between the first beam B 1 and the second beam B2, these two transmission peak patterns are shifted with respect to each other in dependence on the angular displacement. That is, the etalon filter 404 generates for each beam of the two beams B 1 and B2 a transmission peak pattern as a frequency response, when the two beams Bl and B2 travel trough the etalon filter 404. The frequency response of the etalon filter 404 for a beam depends on the angle of incidence, at which the beam enters the etalon filter 404 and, thus, travels through the etalon filter 404.

Therefore, by controlling or setting the angular displacement AD between the two beams Bl and B2 the shift between the transmission peak patterns generated by the etalon filter 404 may be controlled or set.

Fig. 5 schematically shows the angular displacement between two beams originating of an original laser beam with respect to an etalon filter and, thus, allows describing the terms "angular displacement" of two beams and "angle of incidence" of a beam.

As schematically shown in Fig. 5, the angular displacement AD of the two beams B l and B2 originating from the original laser beam B0 corresponds to the difference between the angle of incidence AOIi of the first beam B 1 and the angle of incidence AOI2 of the second beam B2.

The angle of incidence AOIi of the first beam B l with respect to the etalon filter 504 corresponds to the angle, at which the first beam B l enters the etalon filter 504. In particular, the angle of incidence AOIi of the first beam B l corresponds to the angle between the travel direction of the first beam B 1 and the normal vector N on the surface of the etalon filter 504. The angle of incidence AOI2 of the second beam B2 with respect to the etalon filter 504 corresponds to the angle, at which the second beam B2 enters the etalon filter 504. In particular, the angle of incidence AOI2 of the second beam B2 corresponds to the angle between the travel direction of the second beam B2 and the normal vector N on the surface of the etalon filter 504.

Thus, a beam perpendicularly entering the etalon filter 504, i.e. a beam entering the etalon filter in the direction of the normal vector N on the surface of the etalon filter 504, comprises an angle of incidence of 0°.

Fig. 6 schematically shows an excerpt of an implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention. According to Fig. 6, the two differently polarized beams B 1 and B2 with the predetermined angular displacement AD travel through the etalon filter at different angles of incidence AOIi and AOI2 and are detected by two detection means PD1 and PD2, respectively. In particular, the first beam B 1 filtered and output by the etalon filter 604 is detected by a first detection means PD1 and the second beam B2 filtered and output by the etalon filter 604 is detected by a second detection means PD2. Thus, the first detection means PD1 detects the frequency response of the etalon filter 604 (filter response) for the first beam B l travelling through the etalon filter 604; and the second detection means PD2 detects the frequency response of the etalon filter 604 (filter response) for the second beam B2. As mentioned already above, the frequency response of the etalon filter 604 for a beam corresponds to a transmission peak pattern depending on the angle of incidence of the beam, at which the beam enters the etalon filter 604 (refer to Fig. 7).

Preferably, the two detection means PD1 and PD2 are part of the wavelength monitoring and/or controlling device according to the present invention. In particular, the two detection means PD1 and PD2 are two photodiodes. As mentioned already above, any known detection means configured to detect beams B 1 and B2 may be used for implementing the detection means PD1 and PD2. Fig. 7 schematically shows an example of two transmission peak patterns as a result of the etalon filter of the wavelength monitoring and/or controlling device according to the first aspect of the present invention filtering the two beams Bl and B2 with the predetermined angular displacement. In the graph A of Fig. 7, the transmission peak pattern in form of a solid line corresponds to the transmission peak pattern generated by the etalon filter for the first beam Bl (first transmission peak pattern) and the transmission peak pattern in form of a dashed line corresponds to the transmission peak pattern generated by the etalon filter for the second beam B2 (second transmission peak pattern). That is, the solid line indicates the filter response of the etalon filter for the first beam B 1 and the dashed line indicates the filter response of the etalon filter for the second beam B2. As a result of the predetermined angular displacement between the first beam Bl and the second beam B2, the second transmission peak pattern is shifted with respect to the first transmission peak pattern in dependence on the predetermined angular displacement.

As outlined already above, the m-th transmission peak of a transmission peak pattern generated by the etalon filter for a beam travelling through the etalon filter at an angle of incidence may be defined by the following formula:

c

v_m = m

2nl cos(AOI) Therein," V_m" corresponds to the m-th transmission peak of a transmission peak pattern generated by the etalon filter; "c" corresponds to the speed of light; "n" corresponds to the refractive index of the etalon filter; "/" corresponds to the thickness of the etalon filter and "AO I" corresponds to the angle of incidence of the beam travelling through the etalon filter, for which the transmission peak pattern is generated by the etalon filter.

Graph B in Fig.7 displays the feedback signals generated by the beam Bl and B2 filtered by the corresponding transmission peak patterns for an example range of frequency. In particular, the first and second transmission patterns IPDI and IPD2 correspond to detection signals IPDI and IPD2 detected by two detection means PD1 and PD2, in particular photodiodes, of the wavelength monitoring and/or controlling device. A combination of the detected signals IPDI and IPD2 results in feedback with nonzero derivative over the capture range for the frequency of the original laser beam (received from the laser source) with respect to a target frequency frARGET in full range of interest, wherein the target frequency frARGET corresponds to the desired frequency of the laser beam output by the laser source. Therefore, the wavelength monitoring and/or controlling device is suitable for frequency deviation measurement, wherein the target frequency frARGET is not limited to any fixed grid.

Fig. 8 schematically shows a further implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention. The wavelength monitoring and/or controlling device 801 according to Fig. 8 comprises a first beam splitter 803, preferably in the form of a birefringent wedge; and an etalon filter 804. The above description regarding the wavelength monitoring and/or controlling device, in particular the first beam splitter and etalon filter, are also valid for the wavelength monitoring and/or controlling device 801 of Fig. 8, in particular the first beam splitter 803 and etalon filter 804.

The wavelength monitoring and/or controlling device 801 according to Fig. 8 further comprises a second beam splitter 805; two optional polarization filters PFl and PF2; three detection means PD 1 , PD2 and PD3, preferably in the form of photodiodes; an input means 806 and an optional lens 807.

The second beam splitter 805 is configured to guide the two beams B 1 and B2 output and filtered by the etalon filter 804 to the two detection means PDl and PD2, respectively.

The two optional polarization filters PFl and PF2 are configured to let electromagnetic radiation of only one polarization type through. In particular, a first polarization filter PFl of the two filters, which is arranged in the optic path provided by the second beam splitter 805 for guiding the first beam Bl to a first detection means PDl of the three detection means, is configured to let through only electromagnetic radiation of the polarization type of the first beam Bl. Correspondingly, a second filter PF2 of the two filters, which is arranged in the optic path provided by the second beam splitter 805 for guiding the second beam B2 to a second detection means PD2 of the three detection means, is configured to let through only electromagnetic radiation of the polarization type of the second beam B2.

That is, the first polarization filter PFl is configured to let only the first beam Bl travel to the first detection means PDl, as the first polarization filter PFl is transmissive only for electromagnetic radiation polarized in the same way as the first beam B 1 ; and the second polarization filter PF2 is configured to let only the second beam B2 pass to the second detection means PD2, as the second polarization filter PF2 is transmissive only for electromagnetic radiation polarized in the same way as the second beam B2.

According to figure 8, the original laser beam B0, received from a laser source via an input polarization maintaining fiber and the input means 806, comprises a polarization of preferably 45°. The first beams splitter 803 is preferably a birefringent wedge that divides the original laser beam into the two differently polarized beams B 1 and B2, in particular into the first beam Bl with a "fast" polarization and the second beam B2 with a "slow" polarization. The terms "fast" and "slow" are used in the context of beam splitters made of birefringent material, such as a birefringent wedge, to refer to the two polarization components of the original laser beam BO with the polarization of 45°, which two polarization components experience different effective refractive indices in the birefringent material. The second beam B2 with the so called "slow" polarization is the component for which the birefringent material has the higher effective refractive index, i.e. the second beam B2 travels through the birefringent material of the first beam splitter 803 at a slower phase speed. The first beam Bl with the so called "fast" polarization is the component for which the birefringent material has the lower effective refractive index, i.e. the first beam Bl travels through the birefringent material of the first beam splitter 803 at a faster phase speed.

The first and second detection means PD1 and PD2 are configured to detect the first and second beam B 1 and B2, respectively, after the first and second beams B 1 and B2 have travelled through the etalon filter 804, i.e. after they have been filtered and output by the etalon filter 804. As shown in Fig. 8, the optional first and second polarization filter PF1 and PF2 are assigned to the first and second detection means PD1 and PD2, respectively. Thus, the first detection means PD1 receives only electromagnetic radiation of the first beam B l, as the first polarization filter PF1 allows only electromagnetic radiation of the first beams' Bl polarization type to travel through itself to the first detection means PD1; and the second detection means PD2 receives only electromagnetic radiation of the second beam B2, as the second polarization filter PF2 allows only electromagnetic radiation of the second beams' B2 polarization type to travel through itself to the second detection means PD2.

The first beam splitter 803 according to Fig. 8 is configured to split the original laser beam received from an external laser source into the two differently polarized beams B 1 and B2 with the predetermined angular displacement. In addition, the first beam splitter 803 is configured to reflect a part of the original laser beam, preferably 1% to 2% of the original laser beam, to the third detection means PD3. The third detection means PD3 is configured to monitor the power of the original laser beam B0. The optional lens 807 of the wavelength monitoring and/or controlling device 801 is configured to collimate the original laser beam BO received from the external laser source. The input means 806 of the wavelength monitoring and/or controlling device 801 is configured to connect an optical fiber to the device for providing the original light beam from the external laser source, in particular the tunable laser, via the optical fiber to the device. According to Fig. 8 the optical fiber is a polarization maintaining optical fiber. The optional lens 807, first beam splitter 803, etalon filter 804 and second beam splitter

805 are arranged in an optic path starting at the input means 806 of the wavelength monitoring and/or controlling device 801. Thus, the original laser beam B0 received by the input means 806 travels through the lens 807, the first beams splitter 804, the etalon filter 804 (in the form of the two beams B l and B2) and the second beam splitter 805. Since the first beam splitter 803 divides the original laser beam into the two differently polarized beams B 1 and B2 with the predetermined angular displacement, the two beams B 1 and B2 travel in the optic path of the wavelength monitoring and/or controlling device 801 through the etalon filter 804 and the second beam splitter 805. The second beam splitter 805 provides two optic paths, wherein one optic path starting from the second beam splitter 805 comprises the optional first polarization filter PF1 and leads to the first detection means PD1; and the other optic path starting from the second beam splitter 805 comprises the optional second polarization filter PF2 and leads to the second detection means PD2.

The wavelength monitoring and/or controlling device 801 may be packaged into a common optoelectronic package, such as metal or ceramic air cavity hermetic package.

According to Fig. 8, the input laser beam B0 from the input polarization maintaining fiber is collimated by the lens 807 and propagated to the first beam splitter 803, which preferably corresponds to a birefringent wedge element. The input fiber coupled to the input means

806 is preferably polarization maintaining type so that the input laser beam polarization may be controlled and set to preferably 45° to the "fast" axis of the birefringent wedge element 803. The input laser beam power is then split by preferably a ratio of 50:50 into two preferably orthogonally polarized beams Bl and B2 with a controlled angular displacement. The polarization is aligned to the "fast" and "slow" axis of the birefringent wedge element 803 and, thus, the polarization of the first beam Bl is referred to as "fast" polarization and the polarization of the second beam B2 is referred to as "slow" polarization. The etalon filter 804, which preferably is made of glass and/or preferably comprises a 50 GHz FSR, is provided to filter the beams Bl and B2 with the angular displacement, wherein the filter response for the first and second beam is detected by the two detection means PD1 and PD2, which are preferably photodiodes. The polarization filters PF1 and PF2 may be placed in front of the detection means PD1 and PD2, respectively, in order to limit the optical cross-talk of the two beams Bl and B2. The birefringent wedge element 803 may reflect part of the input beam, preferably 1% to 2%, to the third detection means PD3, which may be used as input power monitor.

Fig. 9 schematically shows a further implementation form of the wavelength monitoring and/or controlling device according to the first aspect of the present invention.

Basically, the implementation form of the wavelength monitoring and/or controlling device according to Fig. 9 corresponds to the implementation form of the wavelength monitoring and/or controlling device according to Fig. 8.

Therefore, the above description regarding the implementation form of the wavelength monitoring and/or controlling device according to Fig. 8 is also valid for the implementation form of the wavelength monitoring and/or controlling device according to Fig. 9; and in particular differences are described in the following.

Since the original laser beam B0 is provided by a single-mode optical fiber according to Fig. 9, the laser beam B0 received at the input means 906 of the wavelength monitoring and/or controlling device 901 according to Fig. 9 receives a laser beam with a random polarization.

Therefore, the wavelength monitoring and/or controlling device 901 may comprise a polarizer PF3 configured to polarize the original laser beam B0 with a desired polarization, in particular with a polarization of 45°. The polarizer PF3 is preferably arranged in front of the first beam splitter 903 in the optic path of the wavelength monitoring and/or controlling device 901, so that a laser beam BO with a predetermined polarization, in particular with a polarization of 45°, is applied to the first beam splitter 903.

Thus, the polarizer PF3 is provided to ensure proper polarization control over the laser beam BO incident to the first beam splitter 903.

Fig. 10 schematically shows a laser system according to a second aspect of the present invention. The laser system 1008 of Fig. 10 comprises a laser 1002; a wavelength monitoring and/or controlling device 1001 according to the present invention comprising a first beam splitter 1003 and an etalon filter 1004; an optical element 1009 and an optional controller 1010 for controlling the laser 1002. The wavelength monitoring and/or controlling device 1001 may be implemented as outlined above. Thus, for the description of the wavelength monitoring and/or controlling device 1001, reference is made to the above description of the wavelength monitoring and/or controlling device according to the first aspect of the invention, in particular to the different implementation forms thereof.

Nevertheless, the main function of the wavelength monitoring and/or controlling device 1001 is shown in Fig. 10. Namely, the original laser beam B0 is divided into two differently polarized beams B 1 and B2 with a predetermined angular displacement by the first beam splitter 1003. These two beams B l and B2 with the predetermined angular displacement are then applied to the etalon filter 1004 for filtering, wherein the filter response in form of a transmission peak pattern for the first beam B 1 is shifted with respect to the filter response in form of a transmission peak pattern for the second beam B2 as a result of the predetermined angular displacement. The laser 1002 is preferably a tunable laser and, as already mentioned above, may be implemented in any known way.

The optical element 1009 is configured to guide at least a part of the original laser beam B0, preferably 4% to 5% of the original laser beam B0, from the laser 1002 to the wavelength monitoring and/or controlling device 1001. The rest of the original laser beam is output by the laser system 1008 for providing a laser beam to the outside. The part of the original laser beam B0 guided to the wavelength monitoring and/or controlling device 1001 is for monitoring and/or controlling the actual frequency respectively wavelength of the laser beam output by the laser 1002 and, thus, by the laser system 1008, in order to maintain a desired frequency frARGET of the laser beam.

Therefore, the laser system 1008 may also comprise a controller 1010 configured to control the wavelength respectively frequency of the original light beam B0 emitted from the laser 1002 on the basis of the two light beams B 1 and B2 generated by the first beam splitter 1003 and filtered by the etalon filter 1004 of the wavelength monitoring and/or controlling device 1001. That is, the controller 1010 may control the operation of the laser 1002 on the basis of the monitoring results of the wavelength monitoring and/or controlling device 1001, in particular on the basis of the transmission peak patterns generated by the etalon filter 1004 for the two beams Bl and B2, in order for the laser to provide a laser beam B0 at a desired wavelength respectively frequency f TARGET.

As mentioned already above, the laser system 1008 may also comprise at least two lasers 1002, at least two optical elements 1009 and/or at least two wavelength monitoring and/or controlling devices 1001.

In case the laser system 1008 comprises multiple lasers 1002 and multiple wavelength monitoring and/or controlling devices 1001, the laser system 1008 preferably comprises the same amount of lasers 1002 and wavelength monitoring and/or controlling devices 1001. In such a case, preferably, each of the multiple lasers 1002 is assigned, in particular optically coupled, to one of the multiple wavelength monitoring and/or controlling devices 1001. In particular, each of the multiple lasers 1002 is optically coupled via an optic element 1009 to one of the multiple wavelength monitoring and/or controlling devices 1001.

The controller 1010 may then control the multiple lasers 1002. Alternatively, the laser system 1008 may also comprise more than one controller 1010, wherein each controller 1010 may control one laser or a group of lasers. The controller 1010 may also be provided outside the laser system 1008.

The laser system 1108 according to Fig. 11 comprises a laser 1102, an optical element 1109 and a wavelength monitoring and/or controlling devices 1101.

The above description of the laser system according to Fig. 10 is also valid to the laser system 1108 of Fig. 11.

The wavelength monitoring and/or controlling devices 1101 of Fig. 11 basically corresponds to the implementation form of the wavelength monitoring and/or controlling device 901 according to Fig. 9 and, thus, for describing the wavelength monitoring and/or controlling devices 1101 of Fig. 11 reference is made to the above description of the implementation form of the wavelength monitoring and/or controlling device 901 according to Fig. 9.

According to Fig. 11, the wavelength monitoring and/or controlling device 1101, which preferably is a wavelength locker, is co-packaged with the laser source 1102, the wavelength respectively frequency of which has to be controlled. Preferably, they are co- packaged in a gold box.

The optical element 1109, preferably corresponding to a further beam splitter, located on the optical path feeds part of the laser beam B0, preferably 4% to 5%, to the wavelength monitoring and/or controlling device 1101. Preferably, the laser output polarization is parallel or perpendicular to the device plane. Thus, since the laser output polarization is parallel or perpendicular to the device plane, preferably, a polarization rotator is used as the polarizer PF3 to achieve a 45° polarization of the laser beam B0, in particular a 45° polarization of the laser B0 with respect to the "fast axis" of the birefringent material of the first beam splitter 1103. The first beam splitter 1103 preferably corresponds to a birefringent wedge element.

Fig. 12 schematically shows a method according to a third aspect of the present invention. The method according to Fig. 12 for operating a wavelength monitoring and/or controlling device according to the present invention, as described above, comprises the steps SI and S2.

In the first step S 1 the first beam splitter of the wavelength monitoring and/or controlling device divides the original light beam BO into two light beams B l and B2 with a predetermined angular displacement AD, and differently polarizes the two light beams Bl and B2, such that a first light beam B 1 of the two light beams has a first polarization and a second light beam B2 of the two light beams has a second polarization. That is, in the first step SI the original laser beam BO is divided into two beams Bl and B2 with a predetermined angular displacement and the two beams Bl and B2 are differently polarized, such that a first light beam B 1 of the two light beams has a first polarization and a second light beam B2 of the two light beams has a second polarization.

In the second step S2, following the first step SI, the etalon filter of the wavelength monitoring and/or controlling device filters the two polarized light beams with the predetermined angular displacement. That is, in the second step S2 the two polarized beams with the predetermined angular displacements are filtered.

In the light of the above, the basic concept of the present invention consists of a particular arrangement of a wavelength monitoring and/or controlling device, in particular of a wavelength locker, which relies on a first beam splitter, in particular a birefringent wedge, configured to divide an original laser beam received from a laser source, preferably tunable laser, into two differently polarized beams with a predetermined angular displacement. By means of the first beam splitter an incident laser beam is divided in a pair of beams with preferably orthogonal polarization (ordinary and extraordinary polarization) and controlled angular displacement. The beams are then delivered to an etalon filter (e.g. an etalon filter made of glass) and optionally guided by a second beam splitter, preferably a polarization beam splitter, to two photodiodes, which detect the two beams, respectively.

Since the angular displacement of the two beams corresponds to the difference between the angles of incidence, at which the two beams are delivered to the etalon filter, good control over the angular displacement allows achieving a desired shift between the transmission peak patterns generated by the etalon filter for the two beams. This allows an optimal signals combination with nonzero derivative.

The angular displacement of the beams is determined only by the first beam splitter with negligible change with respect to the orientation of the first beam splitter and/or the etalon filter (within few degrees).

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

A wavelength monitoring and/or controlling device, preferably a wavelength locker, for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a preferably tunable laser, the device comprising

a first beam splitter configured to

divide the original laser beam into two beams with a predetermined angular displacement, and

differently polarize the two beams, such that a first beam of the two beams has a first polarization and a second beam of the two beams has a second polarization; and

The device according to claim 1,

wherein the etalon filter is configured to generate for each beam a transmission peak pattern,

wherein, depending on the predetermined angular displacement, the transmission peak pattern generated for the first beam is shifted with respect to the transmission peak pattern generated for the second beam.

The device according to claim 1 or 2, further comprising

two photodiodes configured to detect the two beams filtered by the etalon filter.

The device according to any one of the previous claims,

wherein the first beam splitter is a birefringent wedge.

The device according to any one of the previous claims,

wherein the predetermined angular displacement is set by the shape and/or material of the first beam splitter.

The device according to any one of the previous claims, wherein the predetermined angular displacement corresponds to an angular displacement between 0° and 2°, preferably between 0.1° and 1°, more preferably between 0.1° and 0.5°.

The device according to any one of the previous claims,

wherein the predetermined angular displacement is such that the transmission peak pattern generated by the etalon filter for the first beam is shifted by 25% of the etalon filter's free spectral range, FSR, with respect to the transmission peak pattern generated by the etalon filter for the second beam.

The device according to any one of the previous claims, further comprising a third photodiode configured to monitor the power of the original laser beam, wherein the first beam splitter is configured to reflect a part of the original laser beam, preferably 1% to 2% of the original laser beam, to the third photodiode.

The device according to any one of the previous claims,

wherein the first beam splitter is configured to orthogonally polarize the two beams. The device according to any one of the previous claims,

wherein the first beam splitter is configured such that the original laser beam is divided into the two beams, which pass through the first beam splitter at different phase speeds and thus become differently polarized.

The device according to any one of the previous claims,

wherein the first beam splitter is configured to divide the original laser beam into the first beam with a fast polarization and the second beam with a slow polarization,

wherein for the second beam the material of the first beam splitter has a higher effective refractive index compared to the first beam for which the material of the first beam splitter has a lower effective refractive index.

The device according to any one of the previous claims, further comprising input means configured to connect an optical fiber to the device for providing the original laser beam from the tunable laser via the optical fiber to the device. The device according to claim 12, wherein

the optical fiber is a polarization-maintaining optic fiber configured to provide the original laser beam with a predetermined polarization, preferably with a polarization of 45°, or

the optical fiber is a single mode optical fiber configured to provide the original laser beam without polarization.

The device according to claim 12 or 13, further comprising

a polarizer arranged between the input means and the first beam splitter, wherein the polarizer is configured to polarize the original laser beam, preferably with a polarization of 45°.

Device according to any of claims 12 to 14, further comprising

a lens for collimating the original laser beam provided to the input means of the device.

A laser system, comprising

at least one wavelength monitoring and/or controlling device according to any one of the previous claims,

at least one preferably tunable laser configured to emit the original laser beam, and at least one optical element for guiding at least a part of the original laser beam, preferably 4% to 5% of the original laser beam, from the preferably tunable laser to the wavelength monitoring and/or controlling device.

The system according to claim 16,

wherein the wavelength monitoring and/or controlling device and the laser source are arranged in a gold box.

The system according to claim 16 or 17, further comprising

a controller configured to control the wavelength respectively frequency of the original laser beam emitted from the tunable laser on the basis of the two beams generated by the first beam splitter and filtered by the etalon filter of the wavelength monitoring and/or controlling device. Method for operating a wavelength monitoring and/or controlling device, preferably a wavelength locker, according to any one of the claims 1 to 15 for monitoring and/or controlling the wavelength respectively frequency of an original laser beam received from a preferably tunable laser, the device comprising a first beam splitter and an etalon filter, wherein

the first beam splitter

divides the original laser beam into two beams with a predetermined angular displacement, and

differently polarizes the two beams, such that a first beam of the two beams has a first polarization and a second beam of the two beams has a second polarization; and

the etalon filter filters the two polarized beams with the predetermined angular displacement.