Source: http://www.google.com/patents/US20020071458?dq=5,963,646
Timestamp: 2013-12-11 17:38:50
Document Index: 522566824

Matched Legal Cases: ['art 1', 'art 13', 'art 1', 'art 1', 'art 13', 'art 1', 'art 1']

Patent US20020071458 - Version with markings to show changes made - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Advanced Patent Search | Sign inAdvanced Patent SearchPatentsThe light beam emitted from a variable wavelength light source element 2 is partly directly monitored in a second detector 2 for automatic power control in an APC circuit 14, and is also partly passed through a wavelength filter 5 with the transmittance thereof varying with the wavelength and monitored...http://www.google.com/patents/US20020071458?utm_source=gb-gplus-sharePatent US20020071458 - Version with markings to show changes madePublication numberUS20020071458 A1Publication typeApplicationApplication numberUS 10/012,412Publication dateJun 13, 2002Filing dateDec 12, 2001Priority dateDec 13, 2000Also published asCN1359178A, EP1215832A2, EP1215832A3, EP1215832B1, US6915035Publication number012412, 10012412, US 2002/0071458 A1, US 2002/071458 A1, US 20020071458 A1, US 20020071458A1, US 2002071458 A1, US 2002071458A1, US-A1-20020071458, US-A1-2002071458, US2002/0071458A1, US2002/071458A1, US20020071458 A1, US20020071458A1, US2002071458 A1, US2002071458A1InventorsTakami IwafujiOriginal AssigneeTakami IwafujiExport CitationBiBTeX, EndNote, RefManReferenced by (7), Classifications (22), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetVersion with markings to show changes madeUS 20020071458 A1Abstract The light beam emitted from a variable wavelength light source element 2 is partly directly monitored in a second detector 2 for automatic power control in an APC circuit 14, and is also partly passed through a wavelength filter 5 with the transmittance thereof varying with the wavelength and monitored in A first detector 65. Automatic frequency control is performed in an AFC circuit 15while detecting wavelength variations by using the filter output and the direct monitored output. Thus, the construction is extremely simplified, while reducing the size and cost. Images(12) Claims(21)
PREFERRED EMBODIMENTS OF THE INVENTION [0036] Preferred embodiments of the present invention will now be described with reference to the drawings. [0037]FIG. 1 is a view showing the construction of a first embodiment of the present invention. Referring to the Figure, a variable wavelength light beam source element 2, which is used in the DWDM system, has the structure of a semiconductor laser array 35 as shown in FIG. 4, and thus each of the disclosed prior art techniques it can not receive the rearward light beams uniformly. In addition, the output light beam should be adjusted by a light beam amplifier 37. It is thus necessary to control the output light beam or the like by monitoring the forward light beam. Accordingly, as shown in FIG. 1, the output forward light beam from the variable wavelength light beam source element 2 is used for oscillation wavelength stabilization and output light beam control. [0038] Referring to FIG. 1, the first embodiment roughly comprises a module part 1 and a control part 13. The module part 1 includes a variable wavelength light beam source element 2, a lens 3 for converting the output forward light beam emitted from the variable wavelength light beam source element 2 to a parallel beam, a beam splitter 4 for splitting the parallel beam into split beams as a fiber input beam and a wavelength monitoring beam, an etalon filter 5 for receiving as input light beam a part of the split parallel wavelength monitoring beam from the beam splitter 4, a first optical detector 6 for receiving the parallel beam having passed through the etalon filter 5 and converting the received beam to an electric signal, and a second optical detector 7 for directly receiving the split parallel wavelength monitoring beam from he beam splitter 7 and converting the received beam to an electric signal. [0039] The module part 1 further includes an electronic cooling element 8 for controlling the temperature of the variable wavelength light source element 2, a temperature sensor 9, an optical fiber 10, a light beam oscillator 11 for preventing the return light beam of the parallel beam, made parallel by the lens 3, from the optical fiber 10, a lens 12 for coupling the filter input beam from the beam splitter 4 to an optical fiber 10 and the optical fiber 10. [0040] The control part 13 includes an APC (Automatic Power Control) circuit 14 for maintaining a constant output light signal power level of the variable wavelength light signal source element 2, an AFC (Automatic Frequency Control) circuit 15 for controlling the oscillation wavelength of the variable wavelength light signal source element 2, an ATC (Automatic Temperature Control) circuit 15 for maintaining a constant temperature of the variable wavelength light signal source element 2, and a temperature compensation circuit 17 for compensating for the temperature characteristic of the etalon filter 5. [0041] In the variable wavelength light signal source element 2, usually the oscillation wavelength is dependent on the current injected into the semiconductor laser diode part and the temperature thereof, and the AFC is performed according to the current injected into the semiconductor laser diode or the electronic cooling element 8. The APC is performed according to the current injected into the semiconductor laser diode part or the optical amplifier part. [0042] In this embodiment, the forward light signal emitted from the variable wavelength light signal source element 2 is converted by the lens 3 to a parallel beam, which is then split by the beam splitter 4. One split parallel beam, i.e., wavelength monitoring beam, is partly passed through the etalon filter 5 and inputted to the first optical detector 6 for conversion to an electric signal a1, and is also partly directly inputted to the second optical detector 6 for conversion to an electric signal b1. [0043] The etalon filter 5 has its light permeability changed according to the wavelength of the input light beam, and the electric signal a1 obtained as a result of the photo-electric conversion through it contains oscillation wavelength data of the variable wavelength light signal source element. The electric signal b1 is fed to the AFC circuit 15 and the APC circuit 14, and the electric signal a1 is fed to the AFC circuit 15. The AFC (i.e., wavelength variation compensation control) of the variable wavelength light signal source element 2 is performed by comparing the ratio between the electric signals a1L and b1L with a reference value of wavelength control. The APC is performed by comparison of the electric signal b1L with a reference value of output light signal power level control. [0044] At the time of the system start such as when closing the power supply to the module, however, the electric signals a1 and b1 are not yet present. At this time, the ATC (Automatic Temperature Control) thus is performed according to the output signal from the temperature sensor 9 to stabilize the oscillation wavelength of the variable wavelength light signal source element 2 such that the wavelength substantially becomes a desired value, and subsequently the AFC is made. Thereafter, the AFC, the ATC and further the APC are made. [0045] As shown in FIG. 3, the etalon filter 5 has two parallel optical mirrors, and its loss wavelength characteristic is provided by interference between these two mirrors. FIG. 2 shows the light transmission rate of the etalon filter 5 plotted against the wavelength. The light transmission rate depends on the phase overlap of two light beams having traveled over different optical path lengths. The phase difference depends on the difference between the optical path lengths covered by the two light beams. A parameter representing the optical path length difference is the product nd of the cavity length d and the refractive index n of the filter shown in FIG. 3. [0046] A change in the product nd causes a change in the transmittive wavelength center of the filter. The transmittance peak interval is called FSR (Free Spectral Range). Denoting the velocity of light by c, when a light means is perpendicularly incident on the filter, the FSR is given as: FSR=c/2 nd. [0047] The FSR thus can be set by choosing the thickness d and the refractive index n of the filter. The thickness d and the refractive index n are subject to temperature changes, and the center wavelength variation per 1� C. is 1 pm to 10 pm, that is, it is {fraction (1/100)} to {fraction (1/10)} of the temperature characteristic of the semiconductor laser. [0048] The construction of the example of the variable wavelength light signal source element 2 shown in FIG. 4 is an assumption that the semiconductor laser array 35, the optical synthesis unit 36 and the optical amplifier 37 are monolithically integrated. The wavelength interval of the DWDM which is presently rapidly spreading, is being shifted from 100 GHz to 50 GHz. [0049] Usually, by changing the temperature of the semiconductor laser by 10� C. the oscillation wavelength thereof is changed by about 1 nm. The above variable wavelength light signal source element for the DWDM unit with wavelength interval of 50 GHz, can cover five channels (with wavelength interval of 50 GHz) by temperature controlling one semiconductor laser in a temperature range of �10� C. As shown in FIG. 5, by arraying the semiconductor lasers at an interval of 2 nm, 20 channels can be covered by using four semiconductor lasers, and 40 channels can be covered by using eight semiconductor lasers. [0050] Wavelength stabilization control by using the variable wavelength light signal source element 2, may be made so long as the same transmittance can be detected periodically for each channel by using etalon filter 5 with an FSR of 50 GHz. For the wavelength control of the variable wavelength light signal source element 2, however, temperature control in a range of �10� C. is necessary, and this means that the control is effected by the transmittance temperature characteristic of the etalon filter 5 with respect to the wavelength. [0051] To preclude the effects of the temperature characteristic of the etalon filter 5, the temperature compensation circuit 17 shown in FIG. 1 is used to input the compensation signal from the temperature compensation circuit 17 to the AFC circuit 15. [0052]FIG. 6 shows transmittance characteristics inclusive of the temperature characteristics of the etalon filter 5. In the Figure, it is assumed that one semiconductor laser in the semiconductor laser array of the variable wavelength light signal source element 2 is being driven. In this case, by making temperature control of the element in a range of �10� C. it is possible to cover five channels (at wavelength interval of 50 GHz). It is also assumed that the etalon filter 5 is preset such that the initial preset wavelength λL0 is located at the center of the wavelength range, in which the transmittance of the etalon filter 5 increases or decreases monotonously. The transmittance at this time is denoted by T0. [0053] In wavelength range in the neighborhood of λL0, the transmittance is less than T0 with an oscillation wavelength greater than λL0 and is greater than T0 with an oscillation wavelength less than λL0. The AFC of the variable wavelength light signal source element 2 is performed such that at the time of T0 the ratio a1L/b1L between the currents a1L and b1L from the first and second optical detectors 6 and 7 has a predetermined reference value. [0054] The APC of the output light signal power level also can be performed by controlling the current injected into the optical amplifier 37 such as to make the current b1L from the second optical detector 7 is constant. [0055] By temperature controlling the variable wavelength light signal source element 2 to change the oscillation wavelength from λL0 to λL+2 spaced apart by 100 GHz, i.e., roughly over two valleys, the transmittance is shifted to point A in FIG. 6 due to the temperature characteristic of the etalon filter 5. By further changing the oscillation wavelength by λL+2, the transmittance is further shifted to point B. The temperature characteristic of the etalon filter 5 is changed linearly. Thus, as shown in FIG. 7, a characteristic shown by the opposite straight plot (i.e., dashed plot) to the characteristic shown by the straight plot (i.e., solid plot) connecting the points A and B in FIG. 6 is calculated in the temperature compensation circuit 17, and the calculated data is inputted as a compensation signal to the AFC circuit 15 for wavelength stabilization control of the variable wavelength light signal source element 2 at the time of wavelength variations. It is thus possible to dispose the etalon filter and the variable wavelength light signal source element on the same electronic cooling element, thus reducing the number of component and also the size of the module. [0056] While FIG. 1 shows the embodiment in a functional block diagram, FIG. 8 schematically shows a specific example of the module part 1 shown in FIG. 1. In FIG. 8, parts like those shown in FIG. 1 are designated by like reference numerals. As shown, the lens 3, the beam splitter 4, the first and second optical detectors 6 and 7 and the temperature sensor 9 as well as the variable wavelength light signal source element 21 and the etalon filter 5 are all disposed on the same electronic cooling element 8. Reference numeral 40 designates a module package. [0057] A second embodiment will now be described. FIG. 9 shows the construction of the second embodiment. In FIG. 9, parts like those in FIG. 1 are designated by like reference numerals. In the previous first embodiment, the variable wavelength light signal source element 2 and the etalon filter 5 are disposed on the same substrate or the same electronic cooling element. In this embodiment, as shown in FIG. 9, the variable wavelength light signal source element 2 and the etalon filter 5 are mounted on separate electronic cooling elements 8 and 19, respectively. [0058] While in the first embodiment the deviation of the AFC signal due to the temperature characteristic of the etalon filter 5 is compensated for by the temperature compensation circuit 17, the separate temperature control of the variable wavelength light signal source element 2 and the temperature compensation circuit 17, requires a second ATC circuit 18 a second electronic cooling element 19 and a second temperature sensor 20 in lieu of the temperature compensation circuit 17. On the merit side, however, the temperature characteristic of the etalon filter 5 need not be taken into considerations, and it is thus possible to alleviate the mounting tolerance of the etalon filter 5 and reduce the number of control steps. [0059] In addition, with the AFC independent from the temperature compensation circuit 17, highly accurate wavelength stabilization is obtainable. In the following control method, instead of the temperature compensation circuit the second ATC circuit 18 for controlling the second electronic cooling element is provided for constant temperature control of the etalon filter 5. The AFC and APC are substantially the same as in the first embodiment, and their descriptions are not given here. [0060] A third embodiment will now be described. FIG. 10 shows the construction of the third embodiment. In FIG. 10, parts like those in FIGS. 1 and 9 are designated by like reference numerals. In this embodiment, as shown in FIG. 10, beam splitters 4 and 21 are disposed for splitting the forward light beam emitted from the variable wavelength light signal source element 2. The split light beam from the beam splitter 4 is received by the second optical detector 7, and the split light beam from the other beam splitter 21 is filtered through the etalon filter 5 and then received for detection by the first detector 6. By using the two beam splitters 4 and 21, it is possible to alleviate the mounting tolerance of the optical detectors and suppress wavelength deviation in the optical path. [0061] The control method in this embodiment is the same as in the first embodiment, and its description is not given here. It will be obvious that the same effects are obtainable by using a trapezoidal prism in lieu of the two beam splitters 4 and 21. The actual mechanical mounting status of the various components is the same as shown in FIG. 8. [0062] A fourth embodiment will now be described. In the above first embodiment, the oscillation wavelength should be present in the linear part of the transmittive characteristic of the etalon filter even when temperature controlling the variable wavelength light signal source element 2 for compensating for the temperature characteristic of the etalon filter. This embodiment has a construction permitting the compensation even when the oscillation wavelength is present in a non-linear part of the characteristic of the etalon filter 5. When the points A and B shown in FIG. 6 are present in the non-linear region, the following method is used. [0063] As shown in FIG. 11, two temperature points around temperature point T are labeled by Ti−1 and Ti+1, and analog-to-digital conversion values obtained from the detected voltage values from the first optical detector in correspondence to the individual temperature points are denoted by Y(T), Y(Ti−1) and Y(Ti+1). These values are written in a ROM or the like. Computation is then performed on the written values to obtain a compensation signal, which is inputted to the AFC circuit for the wavelength control. [0064] The compensation signal used is given as: −(Y(Ti−1)+(Y(Ti+1)−Y(Ti−1)�(T−Ti−−1)/((Ti+1)−(Ti−1)). [0065] Thus, by using this compensation signal accurate wavelength control is obtainable even when the temperature characteristic of the etalon filter 5 is large or when the temperature control range of the variable wavelength light signal source element 2 is excessive. [0066] In the above individual embodiments, the variable wavelength light signal source element 2 may be one in which optical modulator is coupled to the output side of an optical amplifier, or one in which a DBR (Distributed Bragg Reflector) laser, a cascade type DFB (Distributed Feedback) laser and an optical amplifier are integrated. [0067] It is not limitative that the variable wavelength light signal source element 2 has the construction as shown in FIG. 4. Also, the light beam to be monitored is not limited to the forward light beam emitted from the element, but it may also be a rearward light beam therefrom. FIG. 12 shows a construction in the latter case. In this case, a first and a second optical detector 6′ and 7′ are disposed together with the beam splitter 4 on the rear side of the variable wavelength light signal source element 2 for receiving and detecting the rearward light signal emitted therefrom. The first and second optical detectors 6′ and 7′ each have an array structure such that the rearward light beam emitted from each of the semiconductor lasers 351 to 354 in the semiconductor laser array 35 shown in FIG. 4 can be received and detected. [0068] As an application of the device in each of the above embodiments of the present invention, a wavelength multiplexed optical transmission system can be constructed by using a plurality of devices as shown above in an optical transmission station such that wavelengths of individual light signals are close to one another and highly accurately controlled. Also, the individual devices may be utilized as back-up light signal source capable of covering multiple wavelengths. Specifically, the wavelength multiplexed optical transmission system using the variable wavelength optical transmitter according to the present invention comprises an optical transmitting station, an optical receiving station and an optical transmission line interconnecting the optical transmitter and receiver. On the transmission line, an optical amplifier for amplifying an attenuated signal is provided. In this system, a plurality of light signals of different wavelengths are transmitted and received between the optical transmitting and receiving stations. [0069] The optical transmitter is the variable wavelength optical transmitter with a built-in wavelength monitor according to the present invention, and includes a plurality of optical transmitters for outputting signal light beams of different wavelengths from optical waveguides (i.e., optical fibers) and optical synthesis unit for wavelength multiplexing the signal light beams. The optical receiving station includes optical receivers for receiving the signal light beams. [0070]FIG. 13 shows an example of such system construction. Referring to the Figure, optical transmitters 51 to 53 are those as described above according to the present invention. From the output light signals transmitted from these optical transmitters, an optical synthesis unit 54 generates wavelength multiplexed signal, which is amplified by an optical amplifier 55 and fed to an optical processing unit 56. In the optical processing unit 56, such processes as add/drop process of light signals and process of multiplexing with a wavelength multiplexed signal from a different optical amplifier are performed. The output light signal from the optical processing unit 56 is transmitted via an optical fiber 57 to an optical de-synthesis unit 58 for de-synthesis to recover the light signals, which are received in optical receivers 59 to 61. [0071] As has been described in the foregoing, according to the present invention the light signal (either forward or rearward) from the variable wavelength light source element is partly directly monitored for the APC, and is also partly monitored through a wavelength filter, which provides different transmittance values for a plurality of wavelengths, and the AFC is performed while detecting wavelength variations by using the filter output and direct monitor output. Thus, the construction is extremely simplified irrespective of increase of the number of multiplexed wavelengths, and it is possible to reduce size and cost of the device. [0072] For further simplifying the construction, the filter is disposed on the cooling element for temperature control, i.e., wavelength control, of the variable wavelength light source element. In this case, a compensating means for inversely compensating the wavelength versus transmittive characteristic of the filter for temperature variations, and further size reduction is obtainable. The compensating means can be dispensed with by independently temperature controlling the variable wavelength light source element and the filter. [0073] Changes in construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the scope of the present invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting. BRIEF DESCRIPTION OF THE DRAWINGS [0022]FIG. 1 is a view showing the construction of a first embodiment of the present invention; [0023]FIG. 2 shows the light transmission rate of the etalon filter plotted against the wavelength; [0024]FIG. 3 shows structure of the etalon filter; [0025]FIG. 4 shows an example of a variable wavelength light beam source element; [0026]FIG. 5 shows an example of oscillation spectrum of the variable wavelength light beam source element; [0027]FIG. 6 shows transmittance characteristics of the variable wavelength light beam source element; [0028]FIG. 7 shows a drawing for explaining the temperature compensation signal; [0029]FIG. 8 shows a concrete structure of the module part 1 shown in FIG. 1; [0030]FIG. 9 is a view showing the construction of a second embodiment of the present invention; [0031]FIG. 10 is a view showing the construction of a third embodiment of the present invention; [0032]FIG. 11 is a view showing the construction of a fourth embodiment of the present invention; [0033]FIG. 12 shows a construction of the module part utilizing the rearward light of the variable wavelength light source element; [0034]FIG. 13 shows a block diagram of optical system based on DWDM system according to an application of the present invention; and [0035]FIG. 14 shows a prior art structure of variable wavelength control device.
Referenced byCiting PatentFiling datePublication dateApplicantTitleUS6693932 *Jun 22, 2001Feb 17, 2004Opnext Japan, Inc.Optical wavelength stabilization circuit, optical transmitter and optical transmission systemUS6915035 *Dec 12, 2001Jul 5, 2005Nec CorporationVariable wavelength optical transmitter output control method therefor and optical communication systemUS7009716 *Mar 15, 2004Mar 7, 2006Electronics And Telecommunications Research InstituteSystem for monitoring optical output/wavelengthUS7418025 *Mar 17, 2003Aug 26, 2008Avago Technologies General Ip Pte LtdWavelength monitoring method and apparatus and method of making sameUS7710581 *Jul 9, 2008May 4, 2010Avago Technologies General Ip (Singapore) Pte. Ltd.Wavelength monitoring method and apparatus and method of making sameUS7983318Sep 4, 2009Jul 19, 2011Fujitsu LimitedOptical semiconductor deviceUS20100246614 *Mar 26, 2010Sep 30, 2010Furukawa Electric Co., Ltd.Wavelength variable light source system* Cited by examinerClassifications U.S. Classification372/23, 372/32, 372/20, 372/34International ClassificationH04J14/00, H04B10/155, H04J14/02, H04B10/06, H04B10/14, H04B10/04, H01S5/0687, H01S5/022Cooperative ClassificationH01S5/02284, H04B10/572, H01S5/0687, H01S5/02248, H04B10/506, H04B10/564, H01S5/4025European ClassificationH04B10/564, H04B10/506, H04B10/572Legal EventsDateCodeEventDescriptionAug 27, 2013FPExpired due to failure to pay maintenance feeEffective date: 20130705Jul 5, 2013LAPSLapse for failure to pay maintenance feesFeb 18, 2013REMIMaintenance fee reminder mailedSep 24, 2008FPAYFee paymentYear of fee payment: 4Dec 12, 2001ASAssignmentOwner name: NEC CORPORATION, JAPANFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:IWAFUJI, TAKAMI;REEL/FRAME:012377/0931Effective date: 20011207Owner name: NEC CORPORATION 7-1, SHIBA 5-CHOME, MINATO-KUTOKYOFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:IWAFUJI, TAKAMI /AR;REEL/FRAME:012377/0931RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google