External cavity laser with single mode-hop-free tuning

External cavity lasers with single mode-hop-free tuning are generally described. In an example, an external cavity tunable laser system includes an external cavity, a substrate, a chirped grating reflector, and a tunable filter. The substrate has a gain region disposed on the substrate and also includes an active waveguide. The external cavity tunable laser system has a cavity length of the external cavity tunable laser system that is defined by at least a first length of the chirped grating reflector, a second length of the gain region, and a third length of the tunable filter. The cavity length also has an inherent external cavity longitudinal mode. Further, the tunable filter and the chirped grating reflector are configured to synchronize to the inherent external cavity longitudinal mode over a tuning range of the tunable filter.

BACKGROUND

Traditionally, a laser has three main components: an excitation source, an active medium, and a laser cavity. The excitation source is often an energy source such as light, heat, or current. The excitation source excites electrons in the active medium, and upon decay the electrons in the medium emit energy in the form of a light photon. An external cavity laser is a laser with optical elements external to the active medium that either form a laser cavity or form part of a laser cavity. A laser cavity serves as a resonator for the light photons in the cavity. Generally, a resonator is used in a laser to make the light emitted from a laser coherent. If a light beam from a laser is perfectly coherent, all the light will propagate in the same direction, will have the same wavelength, and will have the same phase. This is the ultimate goal of a laser's resonator, though perfect coherence is not generally achievable. In order to form coherent light, the intrinsic properties of a cavity allow only particular wavelengths to resonate in the cavity. If there are photons emitted by the active medium that align with the resonant traits of the cavity, those photons will not continue to propagate within the cavity. While the photons of the particular wavelength of the cavity (and the wavelengths of other integral multiples of the cavity) propagate, constructive interference of the photons boosts the amplitude of the waves within the cavity. The wavelengths emitted by a certain type of active medium are often a function of the type of active medium and the type and intensity of the excitation source. Thus, a laser cavity should be constructed to match the wavelengths emitted by the active medium in order for the laser to function properly.

SUMMARY

In an embodiment, the present technology provides an improved external cavity tunable laser system with single mode-hop-free tuning that includes an external cavity, a substrate, a chirped grating reflector, and a tunable filter. The substrate has a gain region disposed on the substrate and also includes an active waveguide. The external cavity tunable laser system has a cavity length of the external cavity tunable laser system that is defined by at least a first length of the chirped grating reflector, a second length of the gain region, and a third length of the tunable filter. The cavity length also has an inherent external cavity longitudinal mode. Further, the tunable filter and the chirped grating reflector are configured to synchronize to the inherent external cavity longitudinal mode over a tuning range of the tunable filter.

An illustrative method includes generating light in an external cavity. The light has a wavelength and the external cavity has a cavity length. The method also includes amplifying the light at a gain region disposed on a substrate. The method also includes reflecting the light at a chirped grating reflector. The method also includes tuning a tunable filter such that the light is synchronized according to an inherent external cavity longitudinal mode of the external cavity. The external cavity includes a cavity length defined by at least a first length of the chirped grating reflector, a second length of the gain region, and a third length of the tunable filter. The tunable filter and the chirped grating reflector are configured to synchronize to the inherent external cavity longitudinal mode over a tuning range of the tunable filter.

DETAILED DESCRIPTION

Described herein is an external cavity laser that uses a linearly chirped mirror to synchronize an external cavity with a tunable filter, allowing for continuous single mode-hop-free tuning.

FIG. 1depicts a representation of an external cavity laser100that incorporates a chirped mirror110and a tunable filter130in accordance with an illustrative embodiment. The tunable filter130adjusts the cavity length of the external cavity or the wavelengths passing through the cavity (and therefore can adjusts what wavelength is needed to resonate within the cavity or adjust the wavelengths to fit the length of the cavity). Area115roughly demonstrates the length of the tunable external cavity. A gain chip120serves as the active medium and is the source of the stimulated photons introduced into the laser100. The photons generated by the gain chip120pass through a lens150and the tunable filter130and are reflected off of a mirror140, as generally shown along a path165. After reflecting off of the mirror140, the photons pass back through the tunable filter130and the lens150and back through the gain chip120. The light passing through the gain chip120then goes through another lens160and into the chirped mirror110. The chirped mirror110is a mirror having chirped spaces that vary in depth in a manner designed to reflect varying wavelengths of light. In an embodiment, the chirped mirror110may include a plurality of dielectric layers, wherein the chirped spaces are located between the dielectric layers. In this embodiment, the chirped mirror110is a linearly chirped mirror that will reflect certain wavelengths, for example λsmay be the shortest wavelength the chirped mirror110reflects and λLmay be the longest wavelength the chirped mirror110reflects. The chirped mirror110will also reflect the wavelengths between λsand λL. Any wavelength not within that range may not be reflected by the chirped mirror110. As a result, only the appropriate wavelengths within the cavity will be propagated and will allow the wavelengths in the cavity to resonate properly. An emitted laser170is also shown inFIG. 1.

The chirped mirror110can be designed in many ways, but here it is shown in a linear configuration. With a linearly chirped mirror, the wavelengths between λsand λLare reflected back at different locations within the chirped mirror110. At one location in the chirped mirror110wavelengths of λswill be reflected. The location where light is reflected in the chirped mirror110is linearly related to the wavelength of the light. In other words, as the wavelength linearly increases, the physical point in the chirped mirror110where light is reflected increases as well. Using this configuration, the tunable filter130can be adjusted to change the cavity length of the external cavity to match a wavelength being output by the gain chip120. In another embodiment, the tunable filter130may adjust the wavelength of the light in the cavity. As long as the output wavelength is within the range reflected by the chirped mirror110, synchronization can occur between the wavelength and the length of the cavity.

Synchronization is further accomplished because of the configuration of the linearly chirped mirror110. Since the reflection of a particular wavelength happens at a particular point in the chirped mirror110, different cavity lengths will be effected by the chirped mirror110depending on the wavelength that is propagated within the cavity. Thus, when using the chirped mirror110, the optical cavity length adjusts according to the setting of the tunable filter130and the wavelengths being propagated in the cavity. This allows for simple tuning that can be effective over a continuous range without having to physically adjust any component of the external cavity. Additionally, this tuning can be effected without the use of multiple modes to cover a tunable range. Stated another way, the chirped mirror110and the tunable filter130allow for single external cavity longitudinal mode synchronization over a wide range of wavelengths. This system thereby allows realization of continuous single mode-hop-free tuning.

FIG. 2depicts a graph200that demonstrates the relationship between the cavity length and wavelength for achieving synchronization in accordance with an illustrative embodiment. As the wavelength increases, so too must the cavity length in order to achieve synchronization. Thus, using a tunable filter and linearly chirped mirror to effect a variable cavity length, a laser can be tuned to a specific wavelength λ0as depicted. For example, in an illustrative embodiment, the external cavity laser100shown inFIG. 1has the chirped mirror110that can reflect wavelengths between λsand λL. Similarly, the external cavity laser100may also be tuned across that range of wavelengths. When the length of a cavity equals the wavelength of light in the cavity, synchronization is accomplished. (In an alternative embodiment, the length of the cavity may also equal an integral multiple of the wavelength to accomplish synchronization.) Since a chirped mirror, such as the linearly chirped mirror110, reflects different wavelengths at different points in physical space, a laser accomplish synchronization across a range of wavelengths without adjusting any physical component of the laser. That is, the wavelength may change within the range that is reflected by the chirped mirror and the laser will still achieve synchronization. Accordingly, the effective length of the cavity changes along with the wavelength due to the properties of the chirped mirror.

FIG. 2generally shows the linear relationship between cavity length and wavelength for accomplishing synchronization. As the wavelength increases, the so should the cavity length in order to accomplish synchronization. The points at which synchronization might be accomplished is represented by a line205. Accordingly, at a point210where the wavelength is λOand the cavity length is L0synchronization can be accomplished. The embodiments disclosed herein provide a tunable external cavity that can achieve synchronization across a wide range of wavelengths without physically adjusting the cavity.

FIG. 3depicts a representation of an external cavity laser300that incorporates a gain area350and a chirped bragg grating360in the same integrated chip310in accordance with an illustrative embodiment. In this embodiment the gain area350and the chirped bragg grating360are integrated into one physical component. In an illustrative embodiment, the chirped bragg grating360may behave similarly to the chirped mirror discussed above with respect toFIG. 1. In this case, the wavelengths of light generated by the gain area350pass through a lens320and a tunable filter340. The wavelengths are then reflected off of a mirror330and pass back through the lens320and the tunable filter340. The wavelengths will pass back through the gain area350and into the chirped bragg grating360. Here the wavelengths will be reflected by the chirped bragg grating360and propagated back through the external cavity of the external cavity laser300. The output of the laser comes out of the integrated chip310and passes through a lens370.

FIG. 4depicts a graph showing the reflectivity of different chirped mirrors at particular wavelengths in accordance with an illustrative embodiment. This graph is shown in Information Optics and Photonics: Algorithms, Systems, and Applications by T. Fournel and B. Javidi, which is incorporated herein by reference. The graph demonstrates the properties of various chirped mirrors in the art. Each line on the graph represents the properties of a different mirror. This graph shows what ranges of wavelength are reflected by each mirror, and to what level there is reflectivity at those wavelengths. For example, line410shows a mirror with a high relative reflectivity (almost 1 on the graph). A higher reflectivity mirror may benefit a laser by making it highly resonant. However, the mirror associated with the line410has a relatively (compared to the other lines on the graph) narrow range of frequencies at which the mirror demonstrates that high reflectivity. Thus, where the mirror associated with the line410may not calibrate over as wide a region of wavelengths. A line420shows a line associated with a different mirror. The line420shows a lower reflectivity than the line410, but shows a much larger wavelength range across which the mirror still has substantial reflectivity.

FIG. 5depicts a graph showing delay in time of the reflection of particular wavelengths in different chirped mirrors in accordance with an illustrative embodiment. Here, a particular mirror has the linear chirped pattern desired for the embodiments shown inFIG. 1andFIG. 3. This graph shows the delay of reflection for varying wavelengths. The delay is measured from the time the light enters the mirror. Thus, the position of reflection is implicated if the delay of reflection for a given wavelength is longer. In other words, different mirrors are configured to have different effective lengths depending on how they reflect back varying wavelengths. As shown inFIG. 5, different mirrors may be configured differently to reflect different ranges of wavelength, reflect them more slowly or quickly, Each of these factors depends on the physical configuration of each mirror. For example, a mirror associated with a line510reflects a relatively narrow band530of wavelengths compared to the other lines inFIG. 5, but delivers higher variability in delay times for the reflected wavelengths. In contrast, a mirror associated with a line520reflects a larger range540of wavelengths, but with relatively less variability in delay times.

FIGS. 6a-6ddepict chirped Bragg grating devices in accordance with illustrative embodiments. The depictionFIG. 6aindicates how different wavelengths are reflected at different points within the mirror or grating device. That is, as the variable z increases, the wavelength reflected increases as disclosed herein.FIG. 6ais a bird's eye view of a chirped Bragg grating device geometry realized with a sidewall modulated photonic nano-wire.

FIGS. 6band 6cshow scanning electron microscope (SEM) micrographs of an apodized section of a chirped Bragg grating device (FIG. 6b) and an unapodized section before silicon oxide (SiO2) cladding deposition (FIG. 6c).FIG. 6dshows another SEM micrograph of a chirped Bragg grating device with a silicon taper.

FIG. 7depicts a representation of an external cavity laser that uses a tunable ring filter in accordance with still another illustrative embodiment. This depiction gives a visual indication of the schematic shown inFIG. 8, discussed below.

FIG. 8depicts a representation of an external cavity laser800that uses a tunable ring filter810in accordance with an illustrative embodiment. In this embodiment, the tunable filter is a tunable ring filter810. With the tunable ring filter810, the photons pass through the rings of the filter. By adjusting the tunable ring filter810, the path length that the photons must follow changes. In other words, the path the light follows can be shortened or lengthened by the tunable ring filter810. The gain section820and chirped grating reflector830(or chirped mirror) operate similar to those inFIG. 1andFIG. 3. Here, the changing cavity lengths are also demonstrated which allow for tuning the laser and achieving synchronization. The cavity length L is defined by three things: 1) the length (Lg) that the light that passes through the chirped grating reflector830, which is dependent on the particular wavelength of the light; 2) the inherent length (La) of the gain section820; and 3) the length (Lf) of the path through the tunable ring filter810, which is also variable. Therefore, both the length of the tunable ring filter810and the length of the chirped grating reflector830can change. In order to achieve synchronization to account for a change in wavelength of the light in the cavity, the combined change of the length (Lf) of the tunable ring filter810and the length (Lg) of the chirped grating reflector830should be adjusted based on the change in wavelength. This is demonstrated byFIG. 9discussed below.FIG. 8also includes a partial mirror840, which allows some light to pass through outside of the laser and reflects some of the light back into the cavity. Accordingly, a tunable laser as demonstrated inFIG. 8may be tuned by adjusting the wavelength of the light in the cavity and/or tuned by adjusting the ring filter810to change the length of the path through which the light travels. Both of these adjustments occur without having to adjust the physical orientation or configuration of the components shown inFIG. 8.

A ring cavity filter tunes wavelengths by virtue of a refractive index change of the ring area cascaded to the gain area when used in combination with a chirped grating reflector. In order to synchronize the phase of the cavity longitudinal mode and the wavelength of the tunable filter over the tuning range, the rate of cavity length change to total cavity length should be proportional to the rate of wavelength change to center wavelength. This change can be factored by the chirp length of the chirped grating reflector to change the position of reflection, thus changing the cavity length. When the optical cavity length equals the wavelength (or an integral multiple of the wavelength) of the light in the cavity, the cavity longitudinal mode and the wavelength of the tunable filter have been synchronized.

In another embodiment, a MEMS (micro-electro-mechanical system) tunable filter may be inserted in the cavity between the semiconductor chip and the mirror. The semiconductor device is integrated with an active gain region and a chirped grating reflector. The MEMS tunable filter (Fabry-Perot) changes wavelength, but the MEMS tunable filter does not change the optical path length during tuning within the device. Accordingly, the chirp of grating reflector can be simply first order or linear in order to meet the condition of mode synchronization. In this embodiment, rather than changing the path length like the ring filter did, the MEMS tunable filter adjusts the wavelength being resonated in the cavity, but the chirped grating still allows for a wider range of cavity lengths and acceptable wavelengths to make the laser work.

If an embodiment is realized in a compact foot print by integrating or monolithically integrating all elements, longitudinal mode separation becomes large enough, thereby eliminating the need for a high finesse of tunable filter. Accordingly, there is no need to use the Vernier effect with two cascaded ring filters or sampled Bragg gratings in order to make stable single lasing. Consequently, one can achieve wide tuning range continuously with a single filter. Also, a large longitudinal mode range is possible with various embodiments disclosed herein. Thus, the range of wavelengths that make a laser work in a single mode is large. In other words, there is increased single mode selectivity, and the increase creates a more stable single mode oscillation because the laser can handle wider ranges of wavelengths in a single mode.

FIG. 9depicts a graph demonstrating the synchronization conditions for an external cavity laser like the one shown inFIG. 8in accordance with an illustrative embodiment. In other words, a change in cavity length (L0) is represented by the change in space a wave is traveling through the grating (Lg(λ)), which is a function of the wavelength, as well as the change in length (Lf) based on an adjustment to a ring filter or MEMS filter. In order to keep a wave synchronized in a laser, any change to the cavity length (L0) must be proportional to a change in wavelength of the light in the cavity.

FIG. 10depicts a graph that demonstrates the relationship between the cavity length and wavelength for achieving synchronization in accordance with an illustrative embodiment. Again, in order to properly synchronize a laser in a cavity, the wavelength (or an integral multiple of the wavelength) must match the cavity length. Accordingly, the systems and methods disclosed herein advantageously provide for a tunable external cavity laser that can automatically adjust the cavity length using a chirped mirror. The systems and methods disclosed herein are further tunable because ring filters or MEMS filters may be used to further adjust a cavity length, and wavelength may also be adjusted. Advantageously, all of these tuning and adjustment methods can be accomplished without changing the location of physical components of the systems disclosed herein. This also aids in the synchronization process, because it is less likely components will be bumped, displaced, or dislocated, which could further complicate the synchronization process. Furthermore, the physical components of a laser as disclosed herein may be more securely constructed and fastened together because the components are not necessarily physically moved in relation to each other. However, in some alternative embodiments that include components disclosed herein, physical movement or calibration of laser components may still occur.

FIG. 11depicts a gain area and chirped mirror in the same semiconductor1100that may be used in an external cavity laser in accordance with an illustrative embodiment. This is an example of a semiconductor chip that may be used in the embodiment shown byFIG. 3. The semiconductor1100includes a contact layer1105with a p-electrode1110disposed on the contact layer1105and an n-electrode layer1140on the bottom of the semiconductor1100.

The semiconductor1100also includes a grating1115where light may be selectively reflected as disclosed herein. The semiconductor1100also includes a waveguide layer1125and a multi-quantum well (MQW)1120. A gain area in the semiconductor1100includes p-InP layers1130and n-InP layers1135.

Other embodiments are set forth in the following claims.