WO2006087041A1

WO2006087041A1 - Rotational disk polarization controller

Info

Publication number: WO2006087041A1
Application number: PCT/EP2005/050747
Authority: WO
Inventors: Wolfgang Lenz
Original assignee: Agilent Technologies, Inc.
Priority date: 2005-02-21
Filing date: 2005-02-21
Publication date: 2006-08-24
Also published as: EP1853959A1

Abstract

The invention relates to an apparatus and to a method of controlling the polarization of an optical signal, comprising the steps of changing the SOP of an incoming optical signal by rotating a disk (14) being located in an optical path (18) of the apparatus and being rotatable around the axis of the optical path (18), wherein said disk (14) comprises a first quarter wave plate (38) and a first linear polarizer (40) optically connected in series with respect to the optical path (18).

Description

47

DESCRIPTION

ROTATIONAL DISK POLARIZATION CONTROLLER

BACKGROUND ART

[0001] The present invention relates to controlling the state of polarization of an optical signal.

[0002] Polarization controlling serves to control the state of polarization of an optical signal to be able to provide an optical signal with a certain state of polarization. These kind of polarized optical signals are used for example for providing polarized laser light for a measurement of a polarization dependent loss (PDL) of a device under test (DUT).

[0003] One well-known method to measure PDL is the Mueller method. For further details of the Mueller method refer to chapter 6 of "Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR₁ Upper Saddle River, New Jersey 07458, 1998", the disclosure of which is incorporated herein be reference. In these measurements normally four states of polarization (SOPs) are used. As these four SOPs it is known to use the four so called standard

SOPs. These standard SOPs are a circular polarized wave and three linear polarized waves at 0°, 45° and 90°. Alternatively it is possible to use appropriate sets of four SOPs, which cover the Poincare sphere. Based on power measurements for each of the four SOPs the so-called Mueller matrix is calculated, a 4x4 matrix describing the polarization dependency of the loss of the DUT.

[0004] In an alternative way it is known to use the well-known Jones method to evaluate the PDL of the DUT. For further details of the Jones method refer to chapter 6 of "Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR, Upper Saddle River, New Jersey 07458, 1998", the disclosure of which is incorporated herein be reference. In the Jones method the SOPs are analysed by a polarization analyser, e.g. by the Agilent EP2005/050747

8509 Polarization Analyzer of the applicant Agilent Technologies. However, to use the Jones method it is necessary to provide three different SOPs at the input of the DUT and to measure the output SOPs. Normally, three linear SOPs at 0°, +120° and -120° azimuth angle are applied.

[0005] The provision of measurement signals each having a defined polarization is normally done by a polarization controller, e.g. by the Agilent 8169 Polarization Controller of the applicant Agilent Technologies. Such polarization controllers are well known in the prior art. They normally comprise a rotatable linear polarizer followed by a rotatable quarter wave plate and a rotatable half wave plate. By rotating these components of the polarization controller it is possible to provide all SOPs which are necessary for the measurement.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the invention to provide improved controlling the state of polarization of an optical signal.

[0007] The object is solved by the independent claims. Preferred -, embodiments are shown by the dependent claims.

[0008] Embodiments of the present invention comprise the perception that the discrete SOPs provided by a polarization controller, e.g. for a measurement of the PDL of a DUT, should have a high precision relative to each other over a large wavelength range, e.g. at least from about 1250 nm up to about 1650 nm. All this should be easy to implement to avoid any deviation caused by a complicated and therefore sensible structure of the polarization controller.

[0009] An advantage of embodiments of the present invention over the known polarization controllers is the fact that the components in the system does not have to be positioned with absolute precision, whereas in known polarization controllers both the quarter wave plate and the half wave plate have to be positioned and adjusted with great care because of the high 50747

sensibility of the function of both the quarter wave plate and the half wave plate on any maladjustment. According to embodiments of the present invention it is sufficient to have a high relative precision of the components in the system to ensure that the discrete SOPs provided by the polarization controller have a high precision relative to each other over a large wavelength range.

[0010] Embodiments of the present invention further comprise the perception that one of the main drawbacks of the prior art, i.e., the high sensibility of the function of both a quarter wave plate and a half wave plate on any misadjustment can be easily avoided by putting optical components for changing the SOP of a provided circular polarized optical signal in substantially fixed spatial relation to each other.

[0011] Another advantage of embodiments of the present invention over the known polarization controllers is therefore the low dependency of the effect of the system on the wavelength of the initial optical signal, whereas known polarization controllers show a strong dependency of both the effect of the quarter wave plate and the effect of the half wave plate on the wavelength of the initial optical signal.

[0012] In a preferred embodiment of the present invention the system of optical components comprises at least a first quarter wave plate and a first linear polarizer as optical components. Preferably, the first linear polarizer is directly attached to the first quarter wave plate. By attaching the first linear polarizer to the first quarter wave plate, e.g. by using an adhesive for attaching, it is easily possible to adjust the transmission axis of the first linear polarizer in the desired position relative to the fast axis of the first quarter wave plate. Preferably, the fast axis of the first quarter wave plate is being tilted relative to the transmission axis of the first linear polarizer by about 45°.

[0013] By providing a circular polarized optical signal at the input in an embodiment of such an optical system it is easily possible to achieve at an output of the optical system a linear polarized optical signal with an high extinction ratio over a large wavelength with minimum optical insertion loss of the optical system over a large wavelength range. Therefore, the first quarter wave plate does not have to be high end, because the linear SOP of the optical signal at the output is defined by the linear polarizer. On the other hand, the requirements of the first polarizer regarding its extinction ratio can be decreased because an optical input signal of the first polarizer is preferably at least elliptically polarized with an azimuth angle aligned to the transmission axis of the first polarizer.

[0014] In further preferred embodiments the present invention comprises a system of optical components which system comprises an opening for leaving the SOP of the provided circular polarized optical signal unchanged as to provide a circular polarized optical signal when the opening is rotated into the path of the provided circular polarized optical signal. By using such an opening it is easy to provide a circular polarized optical signal, which is unchanged with respect to the initial circular polarized optical signal when rotating the inventive system through the path of the provided circular polarized optical signal.

[0015] In further preferred embodiment the SOP of an initial optical signal is influenced to provide a circular polarized optical signal. Preferably this is achieved by influencing the SOP of an initially linear polarized optical signal, which is polarized along an initial linear polarization axis by using a second quarter wave plate to provide the circular polarized optical signal.

[0016] In another preferred embodiment of the present invention the second quarter wave plate can comprise a, preferably achromatic, Fresnell- prism. Such a prism can be installed stationary so that therefore the installation of such a prism presents no difficulties. Moreover, the effect of the prism is essentially independent on the wavelength of the optical signal. Therefore, this embodiment avoids the strong wavelength dependency of half wave plates known from above mentioned the prior art polarization controllers.

[0017] In another embodiment of the present invention the SOP of an initial 47

optical signal is influenced to provide an initially linear polarized optical signal which is to be provided to the second quarter wave plate to provide the circular polarized optical signal. For influencing the SOP of the initial optical signal it is used a second linear polarizer to provide the initially linear polarized optical signal which is then polarized along an initial polarization axis. Preferably, the initial linear polarization axis is about 45°.

[0018] In a preferred embodiment of the present invention the second quarter wave plate, e.g. the achromatic Fresnell-prism, is positioned about 45° relative to the polarization axis of the second linear polarizer. By using such a position of the polarization axis of the second linear polarizer, it is provided a highly precise circular polarized optical signal to the system of optical components.

[0019] An example of the invention comprises a, preferably conical, detent, which is stationary provided at the certain position to releasable stop the rotation of the system in a certain position by enabling a coupling engagement between the system and the detent.

[0020] Embodiments of the invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. Software programs or routines are preferably applied to the realization of the inventive method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s). EP2005/050747

Fig. 1 and 2 show schematic illustrations of an apparatus according to an embodiment of the present invention.

[0022] Referring now in greater detail to the drawings, Fig. 1 shows a schematic illustration of an apparatus according to an embodiment of the present invention. Apparatus 1 comprises a fiber connector 2, which is connected to a not-shown tunable laser for generating an initial optical signal. Fiber connector 2 provides the initial optical signal to a polarization maintaining fiber 4. The polarization maintaining fiber 4 is connected to a collimator 6, which receives the initial optical signal from the polarization maintaining fiber 4.

[0023] Collimator 6 collimates the initial optical signal and provides the initial optical signal to a second linear polarizer 8 (linear polarizer 8 is called second linear polarizer 8 to facilitate the readability of this description compared with the appended claims). Second linear polarizer 8 has a polarization axis of 45° which polarization axis is indicated by arrow 10.

[0024] Therefore, the second linear polarizer 8 changes the initial optical signal into an initially linear polarized optical signal showing a polarization axis of 45°. The initially linear polarized optical signal, which is leaving the second linear polarizer 8 is then provided to an achromatic Fresnell-rhomb 12. Fresnell-rhomb 12 serves as a second quarter wave plate 12 (quarter wave plate 12 is called second quarter wave plate 12 here to facilitate the readability of this description compared with the appended claims).

[0025] Second quarter wave plate 12 changes the SOP of the initially linear polarized optical signal into a circular polarized optical signal. The circular polarized optical signal is then provided by the Fresnell-rhomb 12 to a system of optical components. System 14 is rotatable about an axis 16 which axis 16 is parallel to the optical path 18 of the optical signal. The rotatability of system 14 is indicated by an arrow 20 around the axis of rotation 16.

[0026] System 14 comprises a disc 22. Disc 22 is a plate with openings 24, 26, 28, 30, 32, 34 and 36. Opening 24 is located in one half of disc 22 whereas openings 26, 28, 30, 32, 34, 36 are substantially located only in the other half of disc 22 as shown in Fig. 2, also. However, only opening 24 is open. Openings 26, 28, 30, 32, 34 are covered by a first quarter wave plate 38 and a first linear polarizer 40. First quarter wave plate 38 is fixed relative to disc 22. First linear polarizer 40 is fixed relative to first quarter wave plate 38. Therefore, the optical components 38 and 40 of the system 14 have a fixed spatial relation to each other. This is also shown in Fig. 2.

[0027] The fast axis 39 of the first quarter wave plate 38 is tilted relative to the transmission axis 41 of the first linear polarizer 40 by 45° as shown in Fig. 2.

[0028] Openings 26, 28, 30, 32, 34 and 36 are positioned at angles 90°, 60° (= - 120°), 45°, 0°, -45° and -60° (= 120°), respectively, on disc 22. Opening 24 which is not covered by the optical components 38 and 40 is located 180° opposite of opening 32.

[0029] In Fig. 1 disc 22 is rotated about rotation axis 16 in a spatial position in which optical path 18 is located in opening 24. Therefore, according to Fig. 1 the circular polarized optical signal traveling along optical path 18 is not influenced by disc 22 but is left unchanged and provided to a second collimator 42 which collimates the circular polarized optical signal and provides the collimated circular polarized optical signal to a single mode fiber 44, which is connected to a fiber connector 46.

[0030] If not the unchanged circular polarized optical signal is desired but another, linear SOP is desired disc 22 has to be rotated about rotation axis 16 into a desired spatial position in which the desired polarization axis is provided by optical components 38 and 40. According to Fig.2 disc 22 can be rotated in the positions of openings 26, 28, 30, 32, 34 and 36 to provide SOPs of 90°, 60° (= -120°), 45°, 0° , -45° and -60° (= 120°), respectively. [0031] To releasable stopping system 14 there is provided a detent 48 which can be moved parallel to rotation axis 16 according to arrow 50. The conical detent 48 cooperates with bores 27, 29, 31, 33, 35 and 37 which releasable stop disc 22 at the positions of the openings 26, 28, 30, 32, 34 and 36, respectively.

[0032] Since all openings 26, 28, 30, 32, 34 and 36 are covered by a single linear polarizer 40 it is provided a highly precise relative position of the aforementioned SOPs. Since only the relative position of each SOP is relevant for the position of the SOPs it is not necessary to provide a high precision in adjusting the relative position of transmission axis 41 of first linear polarizer 40 relative to the fast axis 39 of quarter wave plate 38. This makes it very easy to mount, e.g. by using an adhesive, the first linear polarizer 40 on first quarter wave plate 38 without the necessity of paying attention to a precise relative position of axis 39 relative to axis 41.

[0033] Therefore, apparatus 1 can serve as a polarization controller for providing a circular polarized optical signal when rotating disc 22 in position 24 or to provide any desired discrete linear polarized status by rotating disc 22 in any of the positions 26, 28, 30, 32, 34, 36 or any position where there is an opening in disc 22 which is covered by optical components 38 and 40. Therefore, such a polarization controller can be used in any measurement of PDL as mentioned in the introduction of this application.

[0034] As an initial optical signal provided to the fiber connector 2 it can be used any optical narrowband signal, e.g. provided by the tunable laser.

[0035] Assuming a linear polarized optical signal in front of the second linear polarizer 8 with an extinction ratio of 15 dB, a negligible wavelength dependency of the second quarter wave plate 12, a standard zero order quartz quarter wave plate as the first quarter wave plate 38 and a first linear polarizer 40 with an extinction ratio of 30 dB, over a wavelength range of from 1250 nm to 1650 nm a maximum simulated insertion loss is <0.12 dB and a maximum angle between the Stokes vector of the ideal SOP and the simulated SOP would be <0.15°.

Claims

005/050747CLAIMS:

1. An apparatus (1 ) for controlling the state of polarization -SOP- of an optical signal, comprising:

an optical input (6) and an optical output (42) constituting an optical path (18),

a disk (14) being located in the optical path (18) and being rotatable around the axis of the optical path (18),

wherein said disk (14) comprises a first quarter wave plate (38) and a first linear polarizer (40) optically connected in series with respect to the optical path (18).

2. The apparatus of claim 1 , wherein a fast axis (39) of the first quarter wave plate (38) is tilted relative to a transmission axis (41) of the first linear polarizer (40) by about 45°.

3. The apparatus of claim 1 or any one of the above claims, wherein the disk (14) comprises an opening (24) that is located such that the optical path penetrates the opening (24), if rotated to a certain position.

4. The apparatus of claim 1 or any one of the above claims, further comprising:

a second quarter wave plate (12) inserted into the optical path (18) between the optical input (6) and the disk (14), so that a linear SOP of the optical signal is transformed into a circular SOP.

5. The apparatus of claim 4, further comprising:

a second linear polarizer (8) inserted into the optical path (18) between the optical input (6) and the second quarter wave plate (12) with a polarization axis (10) of about 45° with respect to the fast axis of the second quarter wave plate (12). 47

6. The apparatus of claim 5, wherein the disk (14) is adapted to be rotated into at least for defined rotational positions so that the apparatus provides at the optical output (42) a circular optical light in a first position, a horizontal polarized light in a second position, a vertical polarized light in a third position and a 45° linear polarized light in a fourth position, if provided with a linear polarized light at the optical input (6).

7. The apparatus of claim 1 or any one of the above claims, further comprising a, preferably conical, detent (48), which is stationary provided at a certain rotational position for releasable stopping the disk (14) by enabling a coupling engagement between the disk (14) and the detent

(48) when the disk (14) is rotated into the certain rotational position.

8. The apparatus of claim 4 or any one of the above claims, whereby the second quarter wave plate (12) is a, preferably achromatic, Fresnell- prism.

9. A method of controlling the polarization of an optical signal, wherein an optical path (18) is constituted by an optical input (6) and an optical output (42) and a disk (14) is positioned into the optical path (18) being rotatablθ around the axis of the optical path (18), whereby said disk (14) comprises a first quarter wave plate (38) and a first linear polarizer (40) optically connected in series with respect to the optical path (18), comprising the step of changing the SOP of an optical signal provided to the optical input (6) by rotating the disk (14).

10. A software program or product, preferably stored on a data carrier, for controlling the steps of claim 9, when run on a data processing system such as a computer.