Patent Description:
Optical fiber amplifiers are commonly used in communication systems. Types of optical fiber amplifiers include Rare Earth Doped Fiber Amplifiers, for example, Erbium Doped Fiber Amplifiers ("EDFAs"). The optical fiber amplifiers are usually pumped by one or more light emitter diode (LEDs) or laser pump sources.

An erbium doped fiber (EDF) is a form of a single-mode fiber, having a core that is heavily doped with erbium. Conventional EDFA's include a pump laser that provides a pump light to the EDF to provide amplification of an input optical signal. For example, when pump light at <NUM> or <NUM> is injected into an EDF, the erbium atoms absorb the pump light, which pushes the erbium atoms into excited states. When stimulated by light beams, for example an input optical signal having one or more wavelengths, e.g., in a C-band (<NUM><NUM>) or an L band (<NUM>-<NUM>), the excited atoms return to a ground or lower state by stimulated emission. The stimulated emission has the same wavelength as that of the stimulating light. Therefore, the optical signal is amplified as it is propagating through the EDF.

An example of an optical amplifier is disclosed in <CIT>. The optical amplifier comprises the features according to the preamble of claim <NUM>.

This specification describes technologies relating to optical amplifiers.

In general, one innovative aspect of the subject matter described in this specification is directed to an optical amplifier according to claim <NUM>.

Preferred embodiments of the optical amplifier are defined in the dependent claims.

According to a preferred embodiment, the optical amplifier further includes a controller configured to receive an electrical signal from the photo detector and to control an output power of the pump source based at least in part on the received electrical signal.

According to a preferred embodiment, the integrated component attenuates light received from the output port by <NUM> dB or higher.

According to a preferred embodiment, the optical amplifier further includes an isolator and a tap positioned between the input port and the wavelength division multiplexer. Preferably, wavelength division multiplexer is combined with an isolator forming a third integrated component.

Preferably, the optical amplifier further includes a controller configured to receive a first electrical signal from the first photo detector and a second electrical signal received from the second photo detector and to control an output power of the pump source based at least in part on the received first and second electrical signals. preferably, polarization rotator is a Faraday rotator. The input optical fiber and the output optical fiber are held in a tube. An input optical signal received at the input optical fiber is routed to the prism, wherein the prism passes a portion of the input optical signal to the photo detector and reflects a portion of the input optical signal toward the output optical fiber. According to a preferred embodiment, an input optical signal received at the output optical fiber passes through elements of the integrated component such that the optical signal does not pass through the input optical fiber.

Using an integrated component that provides isolation and tapping of a portion of an optical signal reduces the size and number of components for an EDFA. The integrated component can be swapped as a unit from the EDFA making replacement easy. Additionally, the integrated component can be incorporated such that the EDFA is XFP MSA compliant (e.g., <NUM> GB small form factor pluggable) and compatible with an XFP form factor without significant redesign.

<FIG> is diagram of an example prior art erbium doped fiber amplifier (EDFA) <NUM>. The EDFA <NUM> includes a first tap <NUM> coupled to an input optical port configured to receive an input optical signal having one or more wavelengths. The input optical port can be coupled to an optical fiber that couples the EDFA <NUM> to one or more optical components or fibers. The first tap <NUM> separates a small portion of the input optical signal and outputs this tapped portion to a first photo detector <NUM>. The first photo detector <NUM> can be a photodiode and measures an input power of the input optical signal. The remaining portion of the input optical signal is output from the first tap <NUM> to a first isolator <NUM> coupled to the first tap <NUM>.

The first isolator <NUM> is configured to provide transmission of light signals in one direction. Thus, the first isolator <NUM> can block or greatly reduce optical signals passing back toward the input optical fiber. The output of the first isolator <NUM> is coupled to an input of a wavelength division multiplexer (WDM) <NUM>, whose second input is provided by a pump source <NUM>.

The pump source <NUM> can be a pump laser, a light emitting diode, or other light source. In some implementations, the pump source <NUM> receives a control signal from an electrical controller (not shown) for varying an output power of the light emitted by the pump source <NUM>. For example, the control signal can be a signal for increasing or decreasing the pump light directed toward the WDM <NUM>.

The WDM <NUM> is configured as a combiner that operates to combine the input optical signal and the injected pumping signal provided from the pumping source <NUM> and provides a combined output optical signal to an amplifier fiber <NUM>. In some implementations, the WDM <NUM> combines an input signal having a particular wavelength, e.g., <NUM>, with an injected pumping signal having a different wavelength, e.g., <NUM>. The amplifier fiber <NUM> can be an EDF.

The amplified optical signal output from the amplifier fiber <NUM> is provided as an input to a second isolator <NUM>. The second isolator can be configured to prevent optical signals from passing back toward the amplifier fiber <NUM>. The output of the second isolator <NUM> is coupled to an input of a second tap <NUM>. The second tap <NUM> taps off a small portion of the amplified optical signal and outputs the tapped portion to a second photo detector <NUM>. The second photo detector <NUM> can be a photodiode and used to measure an output power of the amplified optical signal. The remaining portion of the amplified optical signal is output from the second tap <NUM> to an output optical fiber.

The first and second photo detectors <NUM> and <NUM> convert the incident light signals to electrical signals. The electrical signals can be used by the controller to measure the input optical signal power and the amplified output optical signal power. The controller can use the measured input optical signal power and amplified output optical signal power to control performance of the pump source <NUM>.

<FIG> is a diagram of an example EDFA <NUM>. The EDFA <NUM> includes an input port for receiving an input optical signal, e.g., from an input optical fiber. The input port is coupled to a first integrated component <NUM>. The first integrated component <NUM> is configured to provide isolation and signal monitoring functions of an isolator, tap, and photo detector, e.g., first isolator <NUM>, first tap <NUM>, and first photo detector <NUM> of the EDFA shown in <FIG>.

The optical signal output from the first integrated component <NUM> is coupled to an input of a WDM <NUM>, whose second input is provided by a pump source <NUM>.

The pump source <NUM> can be a pump laser, a light emitting diode, or other light source. In some implementations, the pump source <NUM> receives a control signal from an electrical controller (not shown) for varying the output power of the pump source <NUM>. For example, the control signal can be a signal for increasing or decreasing the pump light directed toward the WDM <NUM>.

The WDM <NUM> is configured as a combiner that operates to combine the input optical signal and the injected pumping signal provided from the pumping source <NUM> and provides an output optical signal to an amplifier fiber <NUM>. The amplified optical signal output from the amplifier fiber <NUM> is provided as an input to a second integrated component <NUM>. Similar to the first integrated component <NUM>, the second integrated component <NUM> is configured to provide isolation and signal monitoring functions of an isolator, tap, and photo detector, e.g., second isolator <NUM>, second tap <NUM>, and second photo detector <NUM> of the EDFA shown in <FIG>. The first integrated component <NUM> and the second integrated component <NUM> can each be provided in an integrated package for assembly in an EDFA housing.

<FIG> is a diagram of an example integrated component <NUM>. The integrated component <NUM> includes an input optical fiber <NUM> and an optical output fiber <NUM>. The input optical fiber <NUM> and the output optical fiber <NUM> are packaged in a tube <NUM>. The tube <NUM> can be a glass tube or other suitable material.

The integrated component <NUM> also includes a birefringent crystal <NUM> positioned between the tube <NUM> and a half wave plate <NUM>. The half wave plate <NUM> can be positioned to be along the light path of the input optical fiber <NUM>, but not the light path of the output optical fiber <NUM>. The birefringent crystal <NUM> is configured, for example, to separate incident light having random polarization directions into two orthogonally polarized light beams. A wave plate such as the half wave plate <NUM> rotates a polarization of incident light beams by a specified number of degrees in a particular direction depending on the composition of the wave plate.

<FIG> is a diagram showing relative placement of tube <NUM>, birefringent crystal <NUM>, and half wave plate <NUM> of the integrated component <NUM> of <FIG>. In particular, the birefringent crystal <NUM> is positioned at an end face of the tube <NUM> such that the birefringent crystal <NUM> covers both the input and output optical fibers <NUM>, <NUM> and therefore is in the optical path of light entering or exiting the input optical fiber <NUM> and the output optical fiber <NUM>. However, the half wave plate <NUM> is positioned on top of a portion of the birefringent crystal <NUM> such that the half wave plate <NUM> is only in an optical path of light directed to or from the input optical fiber <NUM> and is not in the optical path of light directed toward the output optical fiber <NUM>.

Returning to <FIG>, the integrated component <NUM> also includes a lens <NUM> positioned between the half wave plate <NUM> and a Faraday rotator <NUM>. The Faraday rotator <NUM> is an optical component that rotates a polarization of light passing through the Faraday rotator <NUM> by a specific amount in response to an applied magnetic field. The lens <NUM> can be used, for example, to focus one or more light beams toward particular optical components, e.g., to focus light on the Faraday rotator <NUM>.

The integrated component <NUM> also includes a prism <NUM> positioned along an optical path between the Faraday rotator <NUM> and a photo detector <NUM>. The prism <NUM> is described in greater detail with respect to <FIG>.

<FIG> is a diagram of the prism <NUM> of the integrated component of <FIG>. The prism <NUM> includes a first end surface <NUM> facing an end surface of the Faraday rotator <NUM>. The first end surface <NUM> of the prism <NUM> has a partial reflective coating that reflects a percentage X of the input optical signal while allowing <NUM>-X percent to pass through the prism <NUM>. In some implementations, the value of X is greater than <NUM> percent.

The prism <NUM> also includes a second end surface <NUM> facing the photo detector <NUM>. The second end surface <NUM> is configured to have a high reflection for wavelengths corresponding to a pump source and low or no reflection for wavelengths of the optical signal. Thus it allows the optical signals reach the photo detector, but prevents the pump signals reach the photo detector. Additionally, in some implementations, the first end surface <NUM> and the second end surface <NUM> are not parallel to each other. In some implementations, instead of the prism <NUM>, a different optical component can be used, for example, an elliptic cylinder with a thin film coating. The thin film coating reflects a portion of incident light and passes a second portion of incident light to the photo detector <NUM>. In some implementations, the coating reflects most of the incident light e.g., greater than <NUM> percent.

Returning to <FIG>, the photo detector <NUM> converts received light signals into electrical signals. The signals can be sent, e.g., to a controller of an amplifier, to calculate a power measurement for the optical signal. For example, the integrated component <NUM> can be positioned to monitor an input optical signal power to an EDFA. In another example, the integrated component can be position to monitor an amplified optical signal output by an amplification fiber of the EDFA.

<FIG> is a side view <NUM> of the integrated component <NUM> of <FIG> on an x-z plane. <FIG> is a top view <NUM> of the integrated component <NUM> of <FIG> on an x-y plane. As shown in <FIG>, the integrated component <NUM> includes tube <NUM> holding an input optical fiber and an output optical fiber, birefringent crystal <NUM>, half wave plate <NUM>, lens <NUM>, faraday rotator <NUM>, prism <NUM>, and photo detector <NUM>. As shown in the side view <NUM>, the birefringent crystal <NUM> and half wave plate <NUM> are not parallel to the end face of the tube or to the z-axis. Additionally, the half wave plate <NUM> is shown as positioned such that light from the input optical fiber passes through the half wave plate <NUM> but light directed toward the output optical fiber does not pass through the wave plate <NUM>.

<FIG> is a side view <NUM> of the integrated component <NUM> of <FIG> on the x-z plane showing polarization states. In particular, the polarization states are shown for light beams passing from the input optical fiber <NUM> to the output optical fiber <NUM>.

A light beam, e.g., an optical signal having one or more wavelength, enters the integrated component <NUM> through the input optical fiber <NUM>. The light beam can be randomly polarized. After the light beam exits the input optical fiber <NUM>, the light beam passes through the birefringent crystal <NUM>.

The birefringent crystal <NUM> separates the optical beam into a first light beam having a first polarization direction and a second light beam having a second polarization direction where the respective polarization directions are orthogonal, as illustrated by box <NUM> showing the polarization state and location for the two beams relative to a cross-section of the integrated component <NUM> following the birefringent crystal <NUM>. As shown in box <NUM>, the light beams have been separated in the upper path along the y-axis.

The first and second light beams are located in the upper portion of a cross section of the integrated device and therefore pass through the half wave plate <NUM> after exiting the birefringent crystal <NUM>. The half wave plate <NUM> rotates the polarization of each light beam by <NUM> degrees clockwise as illustrated by box <NUM>. The first and second light beams pass through the lens <NUM> without any change in polarization as the first and second light beams are directed toward Faraday rotator <NUM>. The Faraday rotator rotates the polarization direction of both the first light beam and the second light beam counter-clockwise by <NUM> degrees as illustrated by box <NUM>.

The first and second light beams exiting from the Faraday rotator <NUM> are directed toward the prism <NUM>. The prism <NUM> has a film coating that passes a first portion of the light beams while allowing second portion to reflect. The first portion passes through the prism <NUM> with the same polarization directions and is focused on the photo detector <NUM>. For example, the passed portion can be a small portion of the light beams incident on the prism <NUM>. The light detected by the photo detector <NUM> can be used, e.g., by a controller of an EDFA, to measure overall optical signal power.

The second portion of the light beams are reflected from the prism <NUM> relative to the y-axis and have the same polarization directions but a relative location mirrored to the lower path of the cross section. The reflected light beams pass back through the Faraday rotator <NUM> where the polarization directions are further rotated by <NUM> degrees counter-clockwise as illustrated by box <NUM> showing both polarization directions and locations for the reflected light beams.

The reflected light beams pass through the lens <NUM> without a change in polarization direction and are then incident on the birefringent crystal <NUM> where, because of the respective locations and polarization directions, the two reflected light beams are merged into one beam exiting the birefringent crystal <NUM>. The merged light beam then enters the output optical fiber <NUM> and exits the integrated component <NUM>.

<FIG> is a side view <NUM> of the integrated component <NUM> of <FIG> on the x-z plane showing polarization states. In particular, the polarization states are shown for light beams passing from the output optical fiber <NUM> toward the input optical fiber <NUM>, i.e., traveling in a reverse direction. However, as illustrated below, the light beam input to the output optical fiber <NUM> is blocked or greatly attenuated, e.g., by <NUM> dB or greater, to limit light exiting the input optical fiber <NUM>.

A light beam, e.g., having optical signals at one or more wavelength, enters the integrated component <NUM> through output optical fiber <NUM>. The light beam can be randomly polarized. After the light beam exits the output optical fiber <NUM>, the light beam passes through the birefringent crystal <NUM>.

The birefringent crystal <NUM> separates the optical beam into a first light beam having a first polarization direction and a second light beam having a second polarization direction where the respective polarization directions are orthogonal, as illustrated by box <NUM> showing the polarization state and location for the two beams relative to a cross-section of the integrated component <NUM> following the birefringent crystal <NUM>. As shown in box <NUM>, the light beams have been separated in the lower path along the y-axis.

The first and second light beams are located in the lower portion of a cross section of the integrated device and therefore do not pass through the half wave plate <NUM> after exiting the birefringent crystal <NUM>. The light beams pass through the lens <NUM> without change in polarization direction and then pass through the Faraday rotator <NUM>. Upon passing through the Faraday rotator <NUM> the polarization directions are rotated <NUM> degrees counter-clockwise as illustrated by box <NUM>.

The light beams exiting the Faraday rotator <NUM> are then incident on the prism <NUM>. A first portion of the light beams is passed through the prism <NUM> while a second portion is reflected. However, due to the output fiber <NUM> position relative to the lens optic axis and the prism <NUM> wedge angle direction, the first portion is directed by the prism <NUM> to exit at an angle that does not provide input to the photo detector <NUM> or provides a very small amount to be detected by the photo detector <NUM>.

The reflected light beams of the second portion are reflected to the upper path of the cross-section of the integrated component <NUM>. Passing back through the Faraday rotator <NUM>, the polarization directions are rotated by a further <NUM> degrees counter-clockwise resulting in positive and negative <NUM> degree polarization directions, respectively as illustrated by box <NUM>. The reflected light beams exiting the Faraday rotator <NUM> pass through the lens <NUM> without changing polarization direction or relative location in the cross-section and exit the lens <NUM> toward the half wave plate <NUM>. After passing through the half wave plate <NUM>, the polarization direction of each reflected light beam is rotated by <NUM> degrees counter-clockwise such that the two light beams again have vertical and horizontal polarization directions, respectively, as shown by box <NUM>.

The reflected light beams enter the birefringent crystal <NUM>. Because of the relative position and the polarization directions, the light beams are not combined by the birefringent crystal <NUM>. Instead, they are further separated such that the exit paths are not incident on the end point of the input optical fiber <NUM>. Thus, the light beams are not passed from the output optical fiber <NUM> to the input optical fiber <NUM>.

<FIG> is a side view <NUM> of the integrated component <NUM> of <FIG> showing a path from the input fiber <NUM> to the photo detector <NUM>. Light entering the input optical fiber <NUM> is able to pass through the birefringent crystal <NUM>, the half wave plate <NUM>, the lens <NUM>, and the Faraday rotator <NUM>. A portion passes through the prism <NUM> depending on a specified transmission rate of the prism <NUM> while the remaining portion is reflected (not shown). The portion that passes through the prism <NUM> is directed to the photo detector <NUM>. The photo detector <NUM> converts incident light to electrical current, which can be used e.g., by an EDFA to determine a power of the light beam entering the integrated component <NUM>.

<FIG> is a side view <NUM> of the integrated component <NUM> of <FIG> showing a path from the output fiber <NUM> to the photo detector <NUM>. Light entering the output optical fiber <NUM> is able to pass through the birefringent crystal <NUM>, the lens <NUM>, and Faraday rotator <NUM>. A portion passes through the prism <NUM> depending on a specified transmission rate of the prism <NUM> while the remaining portion is reflected (not shown). However, the portion passing through the prism <NUM> is now routed such that very little of the light is able to reach the photo detector <NUM>. For example, in some implementations, the electrical current generated by the portion of the light incident on the photo detector <NUM> originating from the input optical port <NUM> is <NUM> times higher than the electrical current generated by the portion of the light incident on the photo detector <NUM> originating from the output optical port <NUM>. This provides uni-direction of the photo detector response.

<FIG> is a diagram of an example one stage EDFA <NUM> including an output monitor. EDFA <NUM> include an input optical fiber <NUM> providing an input optical signal to an integrated WDM and isolator <NUM>. A pump source <NUM> provides a pump light to the integrated WDM and isolator <NUM> such that the WDM combines the input optical signal and the pump light and outputs the combined optical signal to an amplifier fiber <NUM>. The amplifier fiber <NUM>, e.g., and EDF, outputs an amplified optical signal to an integrated component <NUM> providing isolator, tap, and photo detector functions. The integrated component <NUM> is similar to the integrated component <NUM> described above. The integrated component provides output monitoring of the amplified optical signal prior to the amplified output optical signal exiting the EDFA <NUM> from an output optical fiber <NUM>.

Claim 1:
An optical amplifier (<NUM>) comprising:
an input port for receiving an input optical signal;
a wavelength division multiplexer (<NUM>) having a first input coupled to the input port, a second input coupled to a pump source (<NUM>), and an output coupled to an amplification fiber (<NUM>),
a first integrated component (<NUM>) positioned between the input port and the wavelength division multiplexer, the first integrated component configured to provide input monitoring and isolation wherein the first integrated component is configured to: separate a first portion of the input optical signal received from the first port, direct the first portion to a first photo detector and, direct a second portion of the input optical signal to an output coupled to the wavelength division multiplexer, and wherein the first integrated component is configured to attenuate light signals received from the wavelength division multiplexer,
and
a second integrated component (<NUM>) configured to provide output monitoring and isolation, wherein the second integrated component is configured to: separate a first portion of a light signal received from the amplification fiber (<NUM>), direct the first portion to a second photo detector, and direct a second portion of the input light from the amplification fiber (<NUM>) to an output port, and wherein the second integrated component is configured to attenuate light signals received from the output port, and
characterized in that
the first and the second integrated components (<NUM>) each comprising a tube (<NUM>) holding an input and output fiber (<NUM>, <NUM>), followed along an optical path by birefringent crystal (<NUM>), followed by a half wave plate (<NUM>), followed by a lens (<NUM>), a polarization rotator (<NUM>) followed by a prism (<NUM>), followed by a photo detector (<NUM>, and
the prism comprises a first end surface facing the polarization rotator and a second end surface facing photo detector, wherein the first end surface comprises a partial reflective coating, wherein the second end surface has a high reflection for wavelengths corresponding to a pump source and low or no reflection for wavelengths of the optical signal.