TUNABLE OPTICAL FILTER WITH BANDWIDTH TUNING CAPABILITY

Wavelength-tuning optical filters are presented that also allows for the tuning or real-time adjustment of its bandwidth, or passband width. The bandwidth-adjustable tunable optical filters use one or more diffraction gratings that are fixed in place to provide angular dispersion of different wavelengths. A first rotatable or tilting mirror is used to adjust the angle of incidence of an input optical beam to the diffraction grating or diffraction grating system, while a second rotatable or tilting mirror is used to aim the diffracted optical beam back through the diffraction grating or diffraction grating system, so that a subset of the incoming wavelengths are optically aligned to the end face of an output fiber. The first rotatable or tilting mirror provides tuning or adjustment of the bandwidth or passband width of the tunable optical filter, while the second rotatable or tilting mirror tunes or adjusts the center wavelength of the passband.

BACKGROUND

This disclosure relates to tunable optical filters.

Tunable optical filters are a basic building block of modern, reconfigurable optical networks that make use of Wavelength Division Multiplexing (WDM). Tunable optical filters allow rapid reconfiguration of the specific wavelength bands or channels that are being added to, or dropped from, a multi-wavelength optical signal. Tunable optical filters are also used to reduce or eliminate noise, in particular Amplified Spontaneous Emission (ASE) noise, in an optical signal that is sourced from a transmitter or transceiver that incorporates a tunable laser.

Prior art tunable optical filters typically provide a fixed bandwidth, or passband width, while allowing tuning of the center wavelength of the passband. This was perfectly acceptable in optical networks that utilized fixed-bandwidth channels, such as Dense Wavelength Division Multiplexing (DWDM) networks comprising 40 channels with a 100 GHz channel spacing (channel width of approximately 0.8 nm) or 80 channels with a 50 GHz channel spacing (channel width of approximately 0.4 nm). However, with the advent of coherent optics, and advanced photonic modulation schemes, the amount of bandwidth or channel width required for a given channel has become variable, dependent on baud rate, modulation scheme or format, and other variables. In a typical optical networking wavelength band, for example the C band or L band, the channel spacing among wavelength channels in a modern WDM system is not necessarily fixed and different channels may have different bandwidth requirements.

SUMMARY

In one set of embodiments, a tunable optical filter device includes an optical input port, an optical output port, and a diffraction element in an optical path between the input port and the output port, the diffraction element configured to differentially diffract light incident thereupon as based upon wavelength of the incident light. A first rotatable reflector in the optical path is configured to direct at least a portion of light having a wavelength spectrum incident thereupon from the input port to be incident upon the diffraction element, where the angle of incidence upon the diffraction element is dependent on the angle of the first rotatable reflector. A second rotatable reflector in the optical path is configured to direct at least a portion of light incident thereupon from the diffraction element to be incident upon an output port. One or more control circuits are connected to, and configured to independently rotate, the first reflector and the second reflector. The one or more control circuits are further configured to rotate the first reflector to provide the light incident upon the output port to have a selected bandwidth of wavelengths and to rotate the second reflector to align light incident upon the output port to have a selected wavelength center.

In another set of embodiments, a method includes receiving a beam of light at an optical input port; directing, by a first rotatable reflector, at least a portion of the beam of light to be incident on a diffraction element, the diffraction element configured to differentially diffract light incident thereupon as based upon wavelength of the incident light; and directing, by a second rotatable reflector, light diffracted by the diffraction element to be incident on an optical output port. The method also includes rotating, in response to a first user input, the first reflector to provide the light incident upon the output port to have a selected bandwidth of wavelengths; and rotating, in response to a second user input, the second reflector to align light incident upon the output port to have a selected wavelength center.

Various aspects, advantages, features, and embodiments are included in the following description of examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents, and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail.

DETAILED DESCRIPTION

With the advent of coherent optics, and advanced photonic modulation schemes, the amount of bandwidth or channel width required for a given channel has become variable, dependent on baud rate, modulation scheme or format, and other variables. In a typical optical networking wavelength band, for example the C band or L band, the channel spacing among wavelength channels in a modern WDM system is not necessarily fixed and different channels may have different bandwidth requirements. As a result, there is a need for a wavelength-tuning filter that also has the capability of simultaneously and independently adjusting its bandwidth, or passband width. Higher speed optical signals using advanced modulation schemes require more bandwidth, and therefore a wider passband, compared to lower speed optical signals with narrower spectral content. As both types of optical signals may co-exist on a modern WDM network, reconfiguring of the network requires wavelength tuning, as well as tuning or adjusting of the passband width. Similarly, noise filters downstream of a tunable optical transmitter or transceiver may also benefit from tunable or adjustable passband width, if the transmitter or transceiver is capable of supporting a variety of optical signal formats, with different bandwidth requirements. To address this, the following presents embodiments of a wavelength-tuning optical filter that also allows for the tuning or real-time adjustment of its bandwidth, or passband width. The tuning or adjustment of passband width is independent of the tuning of the center wavelength of the passband.

More specifically, the following presents wavelength-tuning optical filters that also allow for the tuning or real-time adjustment of the filters' bandwidth, or passband width. The bandwidth-adjustable tunable optical filters use one or more diffraction gratings that are fixed in place to provide angular dispersion of different wavelengths. A first rotatable or tilting mirror is used to adjust the angle of incidence of an input optical beam to the diffraction grating or diffraction grating system, while a second rotatable or tilting mirror is used to aim the diffracted optical beam back through the diffraction grating or diffraction grating system, so that a subset of the incoming wavelengths are optically aligned to the end face of an output fiber. Alternatively, the second rotatable or tilting mirror may be used to aim the diffracted optical beam at the end face of an output fiber, without first passing back thorough the diffraction grating or diffraction grating system. Thus, the first rotatable or tilting mirror provides tuning or adjustment of the bandwidth or passband width of the tunable optical filter, while the second rotatable or tilting mirror tunes or adjusts the center wavelength of the passband.

FIG. 1shows an example of an optical filter, as described in detail in U.S. Pat. No. 9,720,250. This tunable optical filter uses two diffraction gratings (107and108) that are positioned such that there is an included angle between them. Tuning of the center wavelength of this prior art embodiment is accomplished by changing the angle of a tilting or rotating mirror (131). However, if the two diffraction gratings are fixed in place, the width of the passband is fixed, and is dependent on the overall dispersion coefficient of the two diffraction gratings, the dimensions of the core of the output fiber102, and the length of the optical path between the input fiber (101) and the output fiber (102). The overall dispersion coefficient of the two diffraction gratings is dependent on the included angle.

As is further described in U.S. Pat. No. 9,720,250, if one or both of the diffraction gratings could be made electrically rotatable in order to adjust the dispersion coefficient of the grating system, this would allow adjustment or tuning of the bandwidth or passband width of the tunable optical filter in real-time. However, in practice the diffraction gratings may be too large and heavy to be moved by a typical Micro-ElectroMechanical System (MEMS) mechanism. Mechanical or piezo-electric driven mechanisms are also too bulky for most applications of a tunable optical filter component. Thus, there is a need for a tunable optical filter with real-time adjustment or tuning of bandwidth, that does not require movement of the large and heavy diffraction gratings.

A diffraction grating is an optical component that can split or disperse a beam having multiple wavelengths, into individual wavelength components at different angles. The angular dispersion of the different wavelengths is typically achieved using a periodic structure, such as an array of grooves on the surface of an optical substrate.

FIG. 2Aillustrates the basic working principle of a diffraction grating.FIG. 2Ashows a transmissive diffraction grating202, whose incident beam201and its diffraction beams, represented by211through215, are on opposite sides of the grating surface204. Use of a reflective-type diffraction grating, is shown inFIG. 2B, where the diffracted beams are on the same side as the incident beam, is also applicable and within the scope of the present discussion.

InFIG. 2A, an incoming beam201carrying multiple wavelengths or a continuous wavelength spectrum, is incident on a diffraction grating202, whose perspective view is shown as item220. The beam is therefore diffracted or bent, and split into multiple wavelength components that exit the grating surface204at different angles. The longer the wavelength, the larger the bending angle. For example, ray211has a longer wavelength than that of ray215. Ray213represents a ray whose wavelength is roughly in the middle of the angularly dispersed wavelength spectrum. The relationship between the exiting angle θ and the incident angle α follows the grating equation:

where λ is wavelength and d is the groove distance, or pitch. Thus,

The dispersion coefficient D (the differential of the exiting angle β with respect to wavelength λ) can thus defined by:

Generally, d, the groove distance or pitch, is on the order of the wavelength λ. The smaller the groove distance d is, the greater the dispersion ability. Conversely, the dispersion coefficient decreases with larger groove distance. For smaller or reduced wavelength dispersion, the groove distance d has to be increased. However, when the groove distance d becomes as large as a few multiples of the wavelength, the optical loss resulted from the grating becomes quite polarization dependent, which impairs the optical performance of devices that are built using diffraction gratings. For this and other reasons, a typical diffraction grating that is designed for operation in a wavelength range around 1550 nm will have a groove distance d of about 1 micron, and its dispersion coefficient is therefore around 0.08 degrees/nanometer.

The embodiments described in the following present a grating system that can select a specific wavelength spectrum and also adjust its bandwidth, as shown inFIG. 3. An optical input, here a fiber301, carries a continuous wavelength spectrum as indicated by301A, multiple discrete wavelengths, or a mix of continuous spectrum and discrete wavelengths, and is encapsulated in a ferrule302. Its fiber end311is an optical input port for the tunable optical filter, and is approximately located around the focal point of a lens305. The optical power emitted from the fiber end311is collimated by the lens305to an optical beam denoted by ray307, which is reflected by a first rotatable or tilting mirror317(for example, a MEMS tilt-mirror). The reflected beam330becomes an incident beam330to the diffraction grating325, with an incident angle α. Incident angle α is adjustable, by rotating the mirror317around its pivot point317B by the actuator322, where the mirror317and actuator322can be a MEMs device. The rotation or tilt angle (indicated by319) of the mirror317is controlled by a voltage control circuit321connected to actuator322.

The beam330is then diffracted by the grating325, to form a beam331with an angle (3, with respect to the normal326of the grating325. The beam331is reflected by a second rotatable or tilting mirror345, which rotates around pivot point345B by the actuator342, where the mirror345and actuator342can be a MEMs device. The mirror345is rotated (as indicated by347) by a second voltage control circuit341connected to the actuator342, to an optimal tilt angle, such that the reflected beam337is diffracted a second time by the diffraction grating. The resulting diffracted beam334is then reflected by the first mirror317, to form beam308, which is focused by lens305onto the optical output port, here the fiber end312of the output fiber352.

The voltage control circuit321and voltage control circuit341can receive their respective inputs323and324from a controller320that may be part of one or more control circuits for the optical filter, which can allow a user to select both the wavelength center λcand a bandwidth or passband width for the output signal. The controller320can include a micro-controller (or microprocessor) chip that provides a digital “index value” corresponding to a user input for each of the control signals323and324, to a corresponding DAC (digital to analog converter), which converts the index value to a low-level voltage that can then be amplified to a higher voltage by a corresponding op-amp. Depending on the embodiment, the controller320can be for a single tunable filter, or for one or more tunable filters or other components of an optical system.

The controller320can determine what digital index values to provide based on a translation of the desired filter bandwidth and center wavelength as provided by a user input, into the digital representations of the voltages to be applied to the actuators322and342for the rotatable mirrors317and345. For example, in one set of embodiments the translation can be done by means of a look-up table, stored in either the on-chip memory of the controller320, or in a separate memory device, and the look-up table can be initially created or populated by a calibration process done during manufacture.

The transmission loss of a grating is slightly dependent on the polarization of the incident beam. To avoid polarization dependent loss (PDL) from the input fiber301to the output fiber352, a quarter-wave plate (for example, element360), or a wave plate of some multiple of a quarter wave, may be interposed somewhere between the second mirror345and the grating325so that the polarization of the return beam (337and334) is orthogonal to the incoming beam (330and331). This effectively cancels out PDL for the overall tunable filter.

If a flat wavelength spectrum, such as indicated by301A, propagates inside the optical fiber301, the output spectrum is a narrower wavelength band (as indicated by352A) that has a central wavelength at λcand a bandwidth or passband width BW352B, which is usually defined by the wavelengths at which the optical power is reduced by 3 dB from its spectrum peak.

In summary, the first mirror317is used to change the incident angle α to the diffraction grating, and consequentially the exit angle (3. The change in the incident angle α is twice the change in the mirror's tilt angle. The second mirror345is used to select a central wavelength λcof the portion of the optical signal that enters the core of the output fiber352, similar to the functioning of the single tilt mirror of the prior art tunable optical filter shown inFIG. 1. The incident angle α is not generally at the Littrow angle for the diffraction grating. (The Littrow angle for a diffraction grating is the angle at which the incident angle is equal to the diffraction angle.) The dispersion coefficient of the grating325(as described above) increases with an increase in the incident angle α, and thus the bandwidth of the output spectrum decreases (as explained in more detail below, in relation toFIG. 4). For an incident angle α that is substantially far from the Littrow angle, the transmission loss will be higher than it would be at the Littrow angle.

In the embodiment ofFIG. 3, after reflecting off of the second mirror345, the optical path to the output port (e.g., fiber end312) passes through the diffraction grating325a second time and is then reflected a second time off of the first mirror317, but alternate embodiments can use different geometries. For example, in alternate embodiments the light incident on the second mirror345can be directed to an output port312without a second pass through the diffraction element325; and, in other embodiments, a second pass through diffraction element325can be used, after which the optical path goes to the output port without a second reflection off of the first mirror317. Additionally, althoughFIG. 3illustrates an embodiment where the refractive element is a single transmissive grating325, other refractive elements can be used, including multiple transmissive gratings, or one or multiple reflective gratings, as shown inFIG. 5, and discussed below.

FIG. 4provides a more detailed illustration of the principles of operation of the embodiment shown inFIG. 3. To simplify the illustration, as well as the mathematics, the optical input port (fiber end311ofFIG. 3) and the optical output port (fiber end312) are assumed to be very close to each other, such that the input beam307and the output beam308are almost coincident with each other, as shown by the bidirectional ray401inFIG. 4.

The notations used inFIG. 4are defined below:α0: Incident angle of the multiple incoming wavelengths and the selected central wavelength λ0(also shown as λcin prior sections) to the output port (output fiber);β0: diffracted angle of central wavelength λ0;Δβ: angle deviation of the diffraction angles between the central wavelength and an off-center wavelength;βL: the second incident angle of a reflected off-center wavelength405(βL=β0−Δβ); andαL: the second diffracted angle of an off-center wavelength.

It should be noted that the various rays shown inFIG. 4represent collimated beams. In particular, the reflected beam denoted by ray404is actually a collimated optical beam. Ray405is parallel to Ray404and represents the same collimated beam as ray404. For graphic simplicity, ray405is also used to represent the reflected off-center wavelength.

Based on the grating Equation 1 above, the first diffraction for the central wavelength and the off-center wavelength respectively, are:

The second diffraction for the reflected off-center wavelength is:

Taking Equation 3 minus Equation 2 to the accuracy of the first order leads to:

where Δλ=λL−λ0. Taking Equation 4 minus Equation 3 to the accuracy of the first order leads to:

Equation 7 shows that the dispersion coefficient of a round-trip diffraction depends on the incident angle α0. The output fiber end has a fixed numerical aperture. The greater the wavelength dispersion is, the smaller the bandwidth or passband width of the output spectrum. Therefore, the bandwidth is decreased with increases of the first incident angle α0. Changing the first incident α0by rotating the first mirror317inFIG. 3, results in the selection of the desired bandwidth or passband width BW. The bandwidth at α0=35 degrees is about twice the bandwidth at α0=66 degrees. The Littrow angle is typically in between these two angles.

More than one grating can be used in tandem as a grating system (such as described in U.S. Pat. No. 9,720,250), in order to achieve a dispersion coefficient beyond what a single grating can achieve. However, such a grating system is functionally equivalent to a single grating.FIG. 5shows another embodiment of a bandwidth-adjustable tunable optical filter, comprising two diffraction gratings in tandem. The normal line N1(511) of Grating501, and the normal line N2(522) of the grating502, intersect each other with an included angle θ.

InFIG. 5, the incoming beam541is reflected by the rotating or tilting mirror531to become the incident beam542to the grating501, which diffracts the incident beam542to the beam543. A second grating502diffracts the beam543to the beam544, which goes through a wave plate560and is then reflected by a second rotating or tilting mirror535to become beam545. The return beam545is then diffracted again by the grating502and grating501in sequence to become beam547, which is then reflected again by mirror531to become the output beam548. An output fiber end face (not shown) therefore picks up the optical power carried by the output beam548.

FIG. 6is a flowchart for an embodiment of a method for operating a tunable optical filter device, such as presented above, to provide an output having a selected bandwidth of wavelengths about a selected wavelength center. Referring to the embodiment ofFIG. 3, at step601a beam of light is received from an optical input port, such as the fiber end311of the input optical fiber301. InFIG. 3, the beam of light is collimated by lens305on to the first rotatable reflector or mirror317that, as step603, directs at least a portion of the beam of light to be incident on the diffraction element325, which is a transmissive grating in this example. Light diffracted by the diffraction element325is directed by the second rotatable reflector or mirror345to be incident on the optical output port, such as the fiber end312of output fiber352, as step605. In the embodiment ofFIG. 3, the optical path from the second mirror345to the optical output port makes a second pass through the grating325(and the quarter-wave plate360) and is reflected a second time off the first mirror317, but other embodiments can use different geometries and may lack one or both of the second pass through the grating325and the second reflection off of mirror317.

At step607, the first reflector317can be rotated based on a first user input so that the light incident on the output port (i.e., fiber end312) has a selected bandwidth. In the example ofFIG. 3, this can be based on the control signal323supplied to voltage control circuit I321, which then adjusts the voltage level to the actuator322accordingly. At step609, the second reflector345can be rotated based on a second user input so that the light incident on the output port (i.e., fiber end312) has a selected wavelength center. In the example ofFIG. 3, this can be based on the control signal324supplied to voltage control circuit II341, which then adjusts the voltage level to the actuator342accordingly. Although presented in a particular order inFIG. 6for presentation purposes, when the filter is in use, steps601,603, and605will all be occurring at the same time as part of the filtering process. Steps607and609are independent and can each be performed at any time while the filter is in operation, allowing real time adjustment, or beforehand.

Changing the passband width by rotating the first reflector317will necessitate a different angle of the second reflector345to have the same center wavelength as before. To change only the center wavelength, the second reflector345can be rotated. To change the passband width only while keeping the center wavelength the same, both the first reflector317and second reflector345are rotated as the change in the first reflector317to alter the passband will change the wavelength center as well, which can then be set back to the previous wavelength center by rotating the second reflector345. Consequently, the control signals323and324can change the passband width and center wavelength independently, but changing passband width requires movement of both mirrors.