Tunable optical filter and spectrometer

A tunable optical filter is disclosed having an input port, a beam translator for translating input and output optical beams, an element having optical power for collimating the translated beam, a reflective wavelength dispersive element, and an output port. The beam translator can include a tiltable MEMS mirror coupled to an angle-to-offset optical element. An output port can be extended into a plurality of egress ports, each receiving a fraction of the scanned optical spectrum. A multi-path scanning optical spectrometer can be used as an optical channel monitor for monitoring performance of a wavelength selective switch, or for other tasks.

TECHNICAL FIELD

The present invention relates to optical filters, and in particular to tunable optical filters having an optical beam scanning element.

BACKGROUND OF THE INVENTION

Tunable optical filters are commonly used in optical devices and systems where an optical spectrum of light needs to be modified, with some wavelengths passing through the filter and some wavelengths blocked by the filter. The wavelengths that are passed by the filter form a band of wavelengths called a transmission band. When the filter is tuned, a central wavelength of the transmission band of the filter is tuned. Tunable optical filters are used in optical communications systems to distinguish between optical signals at different wavelengths carrying different information channels. The optical signals at a single central wavelength are called wavelength channels.

A tunable optical filter is frequently used as a key component of an optical spectrometer. In a scanning optical spectrometer, the central wavelength of a tunable filter is scanned, while the detected optical signal is continuously measured. As a result, an optical spectrum of an optical signal is obtained.

Traditional spectrometers are manufactured as laboratory devices which operate under laboratory environmental conditions. A periodic wavelength and optical power calibration is required to ensure the wavelength and power accuracy of these devices. Traditional spectrometers are generally bulky and costly.

Optical communication systems employing wavelength division multiplexing (WDM) technology achieve large transmission capacity by spacing wavelengths of individual wavelength channels as closely as possible, typically less than a nanometer apart. As the channel spacing decreases, monitoring spectral characteristics of the wavelength channels becomes more important. For example, optical channel monitoring can be used to detect an undesirable wavelength drift of wavelength channels.

Optical communication systems employ optical channel monitors (OCM), which function similarly to laboratory grade spectrometers, but are environmentally robust, inexpensive, and compact in size. The spectral resolution and wavelength accuracy of an OCM must be nearly the same as those of a laboratory grade spectrometer, however without requiring extra calibration over the lifetime of the device.

It is preferable that an OCM is capable not only of monitoring all channels in one optical hand, but also monitoring an optical signal-to-noise ratio (OSNR) for each wavelength channel. Today's WDM networks may employ as many as 100 channels at approximately 0.4 nm spacing. The OSNR measurement calls for 0.2 nm spectral resolution or better, at a dynamic range of 40-50 dB.

OCM are used at wavelength routing network nodes to provide straightforward monitoring and alarm-condition recognition. Furthermore, OCM are used to provide “per-channel” optical power measurement for network control loops. Network control loops are thus limited in their response, at least in part, by the rate at which the OCM can update the optical power measurement.

Typically, a network node contains multiple monitoring locations. If there are N monitoring nodes, one brute force approach would be to deploy N OCM, associating one with each monitoring point. This scales the cost, space and power dissipation associated with the aggregate monitoring function by the same factor N. A more common approach is to deploy an N×1 selector switch at an input of a single OCM, considerably reducing the size and cost associated with multi-point monitoring. However, this approach suffers from poorer response time as the aggregate time to monitor all N points is equal to
time=n×(switch settling time+OCM scan time)

An OCM is frequently used to control the attenuation of a wavelength selective switch (WSS) in a reconfigurable optical add/drop node. The optical signal is tapped both before and after the WSS, at each port of the WSS, for the purpose of monitoring. Again, the brute force approach could be to use an independent OCM for each of these locations. Given some means to synchronize the scan of these multiple OCM, the channel power of the input and the output could be measured at one time, thereby giving the information required to calculate the attenuation for each measured channel. However, this approach suffers from aforementioned cost and size disadvantages of deploying multiple OCM. As mentioned previously, a more typical application is to implement a selector switch in front of the OCM to reduce size and cost. In this case, in addition to the considerably slower measurement time, the measurement at input and output ports are no longer made at the same time. To the extent that the power levels being measured are not strictly constant over time, there is some confounding imperfect measurement of the inferred WSS attenuation.

One type of industrial-grade OCM acquires all monitored spectral points of an optical spectrum of an input signal in parallel by dispersing the input light in space and using a photodiode array to simultaneously acquire spectral information at a plurality of monitored frequencies. Disadvantageously, the number of photodiodes in the photodiode array scales proportionally to the number of wavelength channels and spectral resolution, thereby increasing the size and cost of the device and reducing its reliability. If the OSNR of each channel is to be measured, several photodetectors have to be provided within the dispersed light of a single channel. Since current photodiode arrays at telecommunications wavelengths of 1.5 to 1.6 micrometers are often supplied in strips of up to 128 photodiodes, this allows monitoring of just over 30 channels. 512 element arrays are available, which is sufficient for monitoring about 100 channels. However, these arrays are expensive.

Another type of industrial-grade OCM acquires the spectrum by angle-tuning a dispersive element, for example a diffraction grating. U.S. Pat. No. 6,118,530 by Bouevitch et al. teaches a scanning frictionless spectrometer with magnetically actuated flexure-supported diffraction grating and a dedicated separate channel for accurate wavelength referencing during each scan. The advantage of a scanning approach is based on the ability to continuously sweep the wavelength, which greatly improves fidelity of spectra obtained, as well as accuracy of signal-to-noise and peak wavelengths measurements. Detrimentally, a scanning spectrometer is often slower than its detector array based counterpart. A slower measurement speed results from the fact that, in a conventional scanning spectrometer, most of incoming light is discarded, and only a narrow optical frequency component is allowed to impinge on a photodetector at any given time.

An intermediate approach, seeking to benefit from the advantages of both a scanning spectrometer and a spectrometer having a photodetector array, has been disclosed by Onishi et al. in US Patent Application Publication 2007/0177145. Referring toFIG. 1, a spectrometer100of Onishi et al. is presented in a simplified form. The spectrometer100includes an input optical fiber102, a collimating lens104, an acousto-optical deflector (AOD)106controlled by a controller107, a diffraction grating108, a focusing lens110, photodetectors112,113, and114, analog-to-digital converters (ADC)115,116, and117, a signal processing unit118, and a display120. In operation, an input optical signal101exits the optical fiber102at its tip and is collimated by the collimating lens104. A collimated optical beam105is deviated by the AOD106by a controlled variable angle, in dependence on the frequency of a control signal applied by the controller107to the AOD106. The optical beam105is dispersed by the diffraction grating108into individual wavelengths. The photodetectors112to114each detect a fraction of a wavelength-dispersed optical beam111. When the collimated optical beam105is angle-tuned by scanning the AOD control signal frequency, the optical spectrum of the wavelength-dispersed optical beam111is scanned across the photodetectors112to114. Thus, only a fraction of the optical spectrum needs to be scanned across individual photodetectors112to114. The ADC115to117digitize the detected signals and provide the digitized signals to the signal processing unit118. The signal processing unit118then combines the individual fragments of the spectrum and displays the spectrum on the display120.

One disadvantage of the spectrometer100is a reduced reliability, limited scanning range, and high cost. Another is large size, which is detrimental in OCM applications. Yet another disadvantage, which is common to all prior-art spectrometers disclosed above, is that it only has a single optical signal path. Thus, when multiple optical signals need to be measured, multiple OCM have to be used, which increases size and cost of the equipment. Alternatively, 1×N optical selector switch could be used as explained above, which considerably slows down the measurements.

The prior art is lacking an optical tunable filter/spectrometer that would be usable for optical channel monitoring applications while being robust, inexpensive, having a high spectral resolution and a large scanning range, while being capable of monitoring multiple optical signals without sacrificing the measuring time. Accordingly, it is a goal of the present invention to provide such a tunable filter/spectrometer.

SUMMARY OF THE INVENTION

A tunable optical filter of the present invention is capable of operating at high precision and a fast speed, while being scalable to multiple input and output port counts. High fidelity of measured spectra is achieved by using scanning of optical beams. Advantageously, both input and output optical beams are synchronously scanned by the same scanner to improve the wavelength scanning range and/or reduce the required scanning distance or scanning angle. The optical configuration of a tunable filter of the present invention allows scalability by providing multiple signal paths. Furthermore, using arrays of egress optical ports allows one to further reduce the required range of mechanical scanning while improving the light collection efficiency, speed of operation, and reliability. Advantageously, multi-row fiber arrays can be used to ease optical alignment and provide a compact multi-path tunable optical filter that can be used for optical channel monitoring (OCM) applications.

In accordance with the invention there is provided a tunable optical filter comprising:

a first input port for providing a first input optical beam;

a beam translator coupled to the first input port, for translating the first input optical beam in a first plane in response to a control signal applied to the beam translator;

an element having optical power, for collimating the translated first optical beam;

a reflective wavelength dispersive unit coupled to the element having optical power, for angularly dispersing the collimated first optical beam in a plane parallel to the first plane, and for reflecting the wavelength-dispersed first optical beam back towards the element having optical power; and

a first output port.

In operation, the reflected dispersed first optical beam is redirected by the element having optical power back towards the beam translator and translated by the beam translator across the first output port, thereby tuning a central wavelength of the tunable optical filter.

To provide a multi-path capability, the tunable optical filter can include a plurality of input ports for providing input optical beams, including the first input port for providing the first input optical beam, and a plurality of corresponding output ports including the first output port, wherein each input-output port pair defines an optical path of the tunable optical filter. In this embodiment, the beam translator is coupled to the plurality of the input ports, for synchronously translating the input optical beams traveling along the individual beam paths. The translation occurs in planes parallel to the first plane, and the element having optical power is adapted for collimating the translated optical beams. The reflective wavelength dispersive unit angularly disperses the translated optical beams in planes parallel to the first plane and reflects the wavelength-dispersed optical beams back towards the element having optical power. The reflected dispersed optical beams are redirected by the element having optical power back towards the beam translator and synchronously translated by the beam translator across the respective output ports of the plurality of the output ports, thereby synchronously tuning central wavelengths of the individual paths of the tunable optical filter.

In accordance with an embodiment of the invention, there is further provided an embodiment of the tunable optical filter having an array of egress ports disposed in a single plane, wherein the first output port is one of the egress ports. During operation of this tunable filter embodiment, the reflected dispersed first optical beam is translated by the beam translator in the plane of the egress ports along a direction of wavelength dispersion of the dispersed first optical beam, so that the dispersed first optical beam is translated across each egress port, thereby tuning the central wavelength of the tunable optical filter at each one of the egress ports, wherein the central wavelengths at different egress ports of the array of the egress ports are shifted with respect to each other. Having a plurality of egress ports instead of a single output port allows the beam translator to scan over a range that is M times smaller than the full scanning range of a scanner having only one output port, wherein M is the number of the egress ports.

In one embodiment, the tunable optical filter can have a plurality of input ports and a plurality of arrays of egress ports, so that each optical path of the N optical paths is now associated with the M egress ports. In other words, each output port, corresponding to a particular input port of the N input ports, is now an egress port of a particular array of N arrays of M egress ports. Thus, the multipath property of the tunable filter can be combined with the increased speed due to reduced scanning range of the beam translator. In this embodiment, the egress ports of each array are disposed in a single plane. In operation, the reflected dispersed optical beams are translated by the beam translator in the corresponding planes of the arrays of the egress ports, so that each dispersed optical beam is translated across each egress port of one of the arrays of the egress ports, thereby tuning the central wavelength at each egress port. The central wavelengths at different egress ports of a same one of the arrays of the egress ports are shifted with respect to each other, while the central wavelengths of the individual optical filter paths are tuned synchronously.

In any of the above described embodiments of the tunable filter, the linear translator preferably includes a tiltable mirror, such a micro-electromechanical (MEMS) mirror, coupled to an angle-to-offset converting element such as a lens. Further, the element having optical power can be made in a form of a single concave mirror, resulting in a very compact yet efficient device. Waveguide arrays can be used for ease of alignment of the multiple input, output, and/or egress ports.

In accordance with another aspect of the invention, there is provided a spectrometer including any of the tunable optical filters described above, one or more photodetectors coupled to the output ports, and a controller that provides the control signal for the linear translator while collecting photodetector signal(s). In embodiments where the egress ports are used for piecewise spectral scanning as described above, the controller also combines these piecewise spectra to obtain the entire spectra of the input optical signals. Waveguide arrays, including multi-row waveguide arrays, can be used for delivery of light to the photodetectors.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. InFIGS. 2A,2B,FIG. 3,FIGS. 4A,4B,FIG. 5, andFIG. 6A, similar reference numerals denote similar elements.

Referring toFIG. 2A, a spectrometer200of the invention includes an input port202for providing an input optical beam203, a beam translator204for translating the input optical beam203in a plane ofFIG. 2Aby a controllable distance d, a lens206for collimating a translated optical beam207, a reflective diffraction grating208coupled to the lens206, for angularly dispersing a collimated beam209in the plane ofFIG. 2Aand reflecting a wavelength-dispersed optical beam211back towards the lens206, an output port210, a photodetector212, and a controller214for providing a control signal205for the beam translator204. The wavelength-dispersed optical beam211is translated by the beam translator204by the same distance d in an opposite direction to impinge on the output port210and be detected by the photodetector214.

The beam translator204, schematically illustrated inFIG. 2A, can be based on a movable mirror, a translatable motor-driven platform, etc. Preferably, it includes a tillable mirror, such as a galvanometer mirror or a MEMS mirror, and an angle to offset element. This preferred implementation of the beam translator will be considered in detail below, when discussingFIG. 4A.

The scanning operation of the spectrometer200will now be considered. Referring toFIG. 2B, the spectrometer200is redrawn with only central, or so called chief rays215A,215B traced through the spectrometer200, at two magnitudes of beam translation, d1and d2. The chief rays215A and215B are shown in solid and dashed lines, respectively. InFIG. 2B, the chief rays215A and215B represent quasi-monochromatic optical beams. The controller214generates the control signal205for the beam translator204to provide the beam translation of d1, at which the chief ray215A enters the output port210. Then, the controller214detects the photocurrent from the photodetector212. Then, the controller214generates the control signal205for the beam translator204to provide the beam translation of d2. At the beam translation of d2, the chief ray215A misses the output port210. However, a beam at another wavelength, not shown inFIG. 2B, will enter the output port210instead of the beam215B and get detected by the photodetector212. Then, the controller214detects the photocurrent from the photodetector212at the beam translation value of d2. In this manner, the entire optical spectrum of the input optical beam203can be scanned: the controller214applies the control signal205to the beam translator204for translating the input optical beam203, while detecting an output electrical signal220from the first photodetector212, so as to obtain the optical spectrum of the input optical beam203. Notably, the beam translator204translates the chief rays215A and215B twice, once as they exit the input port202and once on their return towards the output ports210. This allows one to effectively double the value of a shift Δd of the chief rays215A and215B at the output port210, thus reducing an amplitude of beam translation required to scan the entire spectrum of the input light beam203. The input port202and the output port210are preferably disposed in a plane forming a non-zero angle with the wavelength dispersion plane, to avoid one of the output optical beams accidentally striking the input port202.

The optical spectrometer200is essentially based on a tunable optical filter230schematically shown by a dotted rectangle inFIG. 2B. The tunable optical filter230includes the input port202, the beam translator204, the lens206, and the diffraction grating208.

Turning now toFIG. 3, a multi-path optical spectrometer300is shown. The multi-path optical spectrometer300is similar to the optical spectrometer200, which is a single-path spectrometer. The multi-path optical spectrometer300is configured to accommodate multiple optical paths therein, enabling simultaneous spectral measurements of a plurality of optical signals, each signal propagating on its own path within the multi-path optical spectrometer300. The multi-path optical spectrometer300includes a multi-path tunable optical filter330, photodetectors312A and312B, and a controller314. The multi-path tunable optical filter includes input ports302A,302B, and output ports310A,310B, defining two independent optical paths within the multi-path tunable optical filter330. The rest of the construction of the multi-path tunable optical filter330is analogous to that of the single-path tunable optical filter230. The multi-path tunable optical filter330includes a beam translator304, a lens306, and a diffraction grating308. The beam translator304is coupled to both input ports302A and302B for synchronously translating input optical beams303A,303B traveling along the corresponding optical paths. The lens306collimates both optical beams303A and303B. The diffraction grating308angularly disperses collimated optical beams309A and309B in planes parallel to the YZ plane, reflecting wavelength-dispersed optical beams311A and311B back towards the lens306. The reflected wavelength-dispersed optical beams311A and311B are redirected by the lens306back towards the beam translator304. The beam translator304synchronously translates the wavelength-dispersed optical beams311A and311B across the respective output ports310A and310B in dependence on the control signal305from the controller314. As a result, central wavelengths of the individual paths of the spectrometer300are tuned synchronously. Of course, the number of input ports, output ports, and/or photodetectors can be bigger than the two shown. When the number of photodetectors is sufficiently large, waveguide arrays and/or photodetector arrays can be used.

In the tunable optical filters230and330, the lenses206and306, respectively, can be substituted for another optical element having optical power (that is, the magnifying power). For example, a concave mirror can be used in place of the lens206or306. Similarly, the diffraction gratings208and308can be substituted for any reflective light-dispersing element or a module. For example, a transmission diffraction grating, coupled with a reflector, can be used in place of the diffraction grating208or308, which effectively doubles the dispersion of the transmission diffraction grating. The beam translators204and304preferably include a tillable mirror coupled to a lens. These components are in fact used in an embodiment of a tunable optical filter described below.

Referring toFIG. 4A, a tunable filter430is similar to the tunable filter330ofFIG. 3. The tunable filter430includes a lens440, a tiltable MEMS mirror442, a concave mirror406, a transmission diffraction grating444, a reflector446, and folding mirrors448and450. Input and output ports402and410are disposed in XZ plane and appear one under the other in the plane ofFIG. 4A(YZ plane). The lens440operates as an angle-to-offset element. Together with the tiltable MEMS mirror442it forms a beam translator404corresponding to the beam translator204ofFIGS. 2A,2B, and the beam translator304FIG. 3. In operation, a diverging input light beam403is collimated by the lens440to form a collimated beam451impinging on the MEMS mirror442. The MEMS mirror442, when tilted, steers a collimated reflected beam453, directing it back to the lens440. The lens440transforms the angle of the collimated reflected beam453into an offset of the refocused beam407which is redirected by the folding mirrors448and450towards the concave mirror406. The concave mirror406collimates the offset beam407to form a collimated beam409impinging on the transmission diffraction grating444. The transmission diffraction grating444angularly spreads the collimated beam409and into beams at individual wavelengths in the YZ plane. Only one such beam455at a center wavelength λCis shown inFIG. 4A. The beam455is reflected back by the reflector446and is diffracted by the diffraction grating444to propagate back through the tunable filter430as a returning light beam411, which retraces back towards the output port410. For each specific angle of the tiltable MEMS mirror442, only a narrow range of wavelengths about a specific wavelength denoted λCfollow the counter-propagating path so as to couple to the output port410, which is symmetrically located about an optical axis of the concave mirror406relative to the corresponding input port402. Other wavelengths, for example those denoted as λi(the full path not shown for visual clarity) are reflected at other angles and thus do not return to the output port410. Adjusting the angle of the tiltable MEMS mirror442changes the incident angle to the diffraction grating444and thus it changes the value of the central wavelength λCof the returning light beam411, which couples to the output port410.

The tunable optical filter430ofFIG. 4can be a multi-path filter where the same set of optics are used to provide a plurality of synchronously tunable optical filters. Each of these synchronously tunable filters has a separate optical path within the tunable filter430. Each optical path is associated with a particular input port and an output port.

A fiber array can be used to provide light input and output to the tunable optical filter430. Referring now toFIG. 4B, a fiber array460including fibers461to466is shown in a frontal view. A tip402A of the third fiber from the left (463) is an input port, and a tip410A of the fourth fiber from the left (464) is an output port of the tunable optical filter430. Together, these define an optical path within the tunable optical filter430. During scanning of the tiltable MEMS mirror442, a wavelength-dispersed focal spot468A is translated across the fiber tip410A in a direction indicated by a “MEMS Tilt” arrow (that is, along Y axis). Similarly, a tip402B of the second fiber from the left (462) is another input port, and a tip410B of the fifth fiber from the left (465) is another output port of the tunable optical filter430, defining another optical path within the tunable optical filter430. During scanning of the tillable MEMS mirror442, a wavelength-dispersed focal spot468B is translated across the output port410B as shown by the “MEMS Tilt” arrow. The wavelength-dispersed focal spots468A and468B are scanned synchronously because the same tiltable MEMS mirror442is used for scanning. A third pair of fibers shown in dashed lines, specifically the first fiber461and the last fiber466, can be used to make a third optical path of the tunable optical filter430. The total number of available optical paths in the tunable optical filter430is limited by the number of fibers in the fiber array460that can be used without introducing too large off-axis optical aberrations.

Separate fiber arrays can be used for the input fibers461-463and the output fibers464-466. Furthermore, planar waveguide arrays can be used instead of fiber arrays. The fibers or waveguides can be concentrated to reduce the fiber/waveguide pitch, so as to accommodate more input and output ports. A microlens array can be placed in front of the fiber or waveguide array to create input beams with larger waist size, which serves to reduce the ratio of fiber/waveguide pitch to beam waist size, this also allows more input and output ports to be accommodated. To make sure that the dispersed focal spots do not overlap, the input and the output ports402and410can be disposed in a plane forming an angle with the plane with the plane of dispersion (YZ plane inFIG. 4A). For example, this angle is 90 degrees inFIG. 4B, placing the input and output ports parallel to the XZ plane, but could be another non-zero angle such as 60 degrees in which case the input and output ports are not parallel to the XZ plane. It is to be understood, however, that the fibers or waveguides are optional, and the tunable optical filter430, as well as the tunable optical filters330and230, can be free space coupled using corresponding input and output ports402A, B and410A, B, respectively. As an example, photodetectors could be placed directly at the locations410A and410B instead of using optical fibers to transmit the signals to external photodetectors.

One of the optical paths of the tunable optical filter430can be used for providing a wavelength reference signal. Referring toFIG. 5, a spectrometer500includes a multi-path tunable optical filter530, photodetectors512A and512B, a reference light source590including a LED591and a Fabry Perot etalon592, and a controller514. The multi-path tunable optical filter530is similar to the multi-path tunable filters430,330, and230ofFIGS. 4A,FIG. 3, andFIG. 2B, respectively. The multi-path tunable optical filter530has two input ports502A and502B and two output ports510A and510B, defining two paths within the tunable optical filter530. In operation, the controller514generates a control signal505for tuning the central wavelength λCof the tunable optical filter530, while detecting photocurrents of the photodetectors512and512B representative of optical signals in the two paths. The reference light source590provides a spectrum having peaks and/or valleys at known wavelengths, to provide wavelength reference points during scanning of the central wavelength λC. A fiber Bragg grating, a gas cell, or any other element with known attenuation spectrum can be used in place of the Fabry Perot etalon592. The reference light source could also comprise a laser or multiple lasers, or a gas discharge lamp emitting known wavelengths.

Turning now toFIG. 6A, a spectrometer600is similar to the spectrometers200,300, and500ofFIGS. 2,3, and5, respectively. The spectrometer600includes a tunable optical filter630, a controller614, and an array of photodetectors682. The tunable optical filter630includes an input port602, a beam translator604, a lens606, and a diffraction grating608. The tunable optical filter630operates similarly to the previously described tunable filters230,330, and430. The distinctive feature of the tunable filter630is that in place of a single output port per single input port, there are plural output ports per single input port. To distinguish these plural output ports from the output ports of the tunable filters330and430described above, the output ports of the tunable optical filter630ofFIG. 6are called herein “egress ports”. Specifically, the tunable optical filter630has not just one output port, but an array of egress ports680, which are disposed in YZ plane. In operation, a reflected dispersed first optical beam611is translated by the beam translator604in YZ plane across the egress ports680, so that a wavelength-dispersed focal spot (not shown inFIG. 6A) is translated across each egress port of the array of egress ports680, thereby tuning the central wavelength λCof the tunable optical filter630at each one of the egress ports680. The translation of the wavelength-dispersed optical beam611is illustrated inFIG. 6Aby means of two chief rays,615A and615B, shown in solid and dashed lines, respectively, corresponding to two different magnitudes of beam translation. Since individual egress ports of the array680are shifted relative to each other, central wavelengths λCEat different egress ports of the array680are shifted with respect to each other as well.

The controller614is configured for applying a control signal605to the beam translator604for translating the input optical beam615A, while detecting output electrical signals620from each photodetector of the array682, so as to obtain partial optical spectra of an input optical signal683from a light source684. Once the partial optical spectra are obtained, the optical spectrum of the input optical signal683can be calculated by combining the obtained partial optical spectra, as described below.

Referring toFIG. 6B, the process of obtaining partial optical spectra is illustrated by means of a diagram690showing the spectrum of the light source684ofFIG. 6Aat two positions denoted as686A and686B. These two positions correspond to two positions of the chief rays615A and615B, respectively, shown inFIG. 6A. Still referring toFIG. 6B, photodetectors682-1,682-2, . . .682-5of the photodetector array682detect optical signals at central wavelengths λCEat corresponding egress ports680-1,680-2, . . .680-5. As the wavelength-dispersed optical beam611shifts from the position of the chief ray615A to the position of the chief ray615B, the spectrum shifts from the position686A to686B, as indicated by an arrow687. As the spectrum shifts, the signals from the photodetectors682-1,682-2, . . .682-5of the photodetector array682are detected by the controller614, so as to obtain partial optical spectra. The whole optical spectrum of the light source684is obtained by combining the partial optical spectra in the controller614. The presence of egress ports680allows one to speed up the scanning because the scanning of the entire optical spectrum across a single output port is no longer required. The efficiency of light collection also improves because less light is discarded during scanning, which improves signal-to-noise ratio of the scanned spectrum. The translation range of the beam translator604is reduced, which in the case of a MEMS mirror reduces the angular tilt range required of the MEMS mirror.

The multi-path tunable optical filters330,430, and530can also be configured to have an array of egress ports per each input port, similarly to the tunable optical filter630described above. The tunable optical filters330,430, and530would then have a plurality of arrays of egress ports, egress ports of each array being disposed in a single plane. In this case, each input port of the plurality of output ports is associated with an array of egress ports, not just one output port. A plurality of arrays of photodetectors will then he required, each array corresponding to an array of the plurality of the arrays of the egress ports. Within each array, one of the photodetectors will need to be coupled to a particular egress port of a corresponding one of the arrays of the egress ports. Each one of the input ports, a corresponding array of the plurality of the arrays of egress ports, and a corresponding array of the plurality of the arrays of the photodetectors together define an optical path of the multipath spectrometers optical filters330,430, and530.

During operation of the multi-path, multiple egress ports variants of the spectrometers300,500, and600, the reflected dispersed optical beams are translated by the corresponding beam translators304,504, and604in the planes of the arrays of the egress ports, so that each dispersed optical beam is translated across each egress port of one of the arrays of the egress ports, thereby tuning the central wavelength at each egress port. The central wavelengths at different egress ports of a same one of the arrays of the egress ports are shifted with respect to each other, while the central wavelengths of the individual optical paths are tuned synchronously. The controllers314,514, or614would need to be configured for applying the control signal to the beam translator304,504, and604, respectively, for translating the input optical beams, while detecting the output electrical signals from each photodetector, so as to obtain partial optical spectra for each of the input optical beams, and to obtain the optical spectrum of each of the input optical beams by combining the corresponding partial optical spectra, in a similar manner as in the spectrometer600described above.

A multi-row fiber array can be used to provide all the required ports in multi-path, multiple egress ports variants of the tunable optical filters330,430,530, and630in a single array. Referring now toFIG. 7, a multi-row array700is presented. For each input fiber1,2,3,4, there is a corresponding vertical row of egress fibers1′,2′,3′,4′. Along each of these rows, a wavelength-dispersed focal spot1″,2″,3″,4″ is translated during tuning of the corresponding tunable filter. For example, when the multi-row fiber array700is used in the tunable optical filter430ofFIG. 4A, the wavelength-dispersed focal spots1″,2″,3″,4″ are translated upon tilting of the MEMS mirror442in a direction indicated by the “MEMS Tilt” arrow inFIG. 7. Other fibers702of the multi-row fiber array700, drawn in dotted line, could be used to provide additional optical inputs, for example, for a scanning range having different start and end wavelengths for the optical spectra of the input optical beams303A and303B of the scanning spectrometer300.

In general, the multi-row fiber array700can have M rows of arrays each having 2N fibers, wherein M is the number of egress ports, and N is the number of input ports. Alternatively, a 1×N array can be used for the input ports, and a M×N multi-row array can be used for the egress ports. Photodetector arrays, one coupled to each egress port by the M×N multi-row array, can be used to save space and simplify the assembly. As in case of the single-row fiber array460ofFIG. 4B, the number of usable fibers in the multi-row fiber array700is limited by optical aberrations in the optics of the tunable optical filters330,430,530, and630. A multi-row waveguide array may be used in place of the multi-row fiber array700.

Turning toFIG. 8, a multi-path spectrometer800includes an embodiment of the multi-path tunable optical filter430having N input ports, a measurement unit802, a microcontroller804, a high-voltage MEMS driver806, and an interface connector808. In one embodiment, the multi-path tunable optical filter has N output optical fibers; in another, it has M×N output optical fibers, that is, M egress-port output fibers for each of the N input fibers. Accordingly, the measurement unit802includes N or M×N photodiodes, N or M×N corresponding trans-impedance amplifiers (TIA), and N or M×N analog-to-digital converters (ADC), as the case may be. The measurement unit802also has RTD electronics for optional temperature compensation.

The measurement data are received by the microcontroller804through a bus805as it controls the scanning voltage generated by the MEMS driver806, for scanning the central wavelength λCof the tunable optical filter430. The scanning is controlled through a control line807. The high voltage for tilting the MEMS mirror442is supplied through a high-voltage line809.

The multi-path optical spectrometers300,500,600, and800can be used for multi-point optical channel monitoring in an optical network. They can be used to monitor optical signals at an input and an output, or outputs of a wavelength selective switch. Since all the spectra are obtained essentially simultaneously as explained above, the fidelity of the measurements of the performance of the wavelength selective switch is improved.

Various embodiments of spectrometers have been considered above. These spectrometers are all based on tunable filters disclosed above. It is to be understood that these tunable filters can also be used in other applications, including signal filtering, adding or dropping wavelength channels in an optical networks, and other applications.

The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many permutations, modifications, and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.