All Polarization-Maintaining, Passively Mode-Locked Linear Fiber Laser Oscillator

An example all polarization-maintaining, passively mode-locked linear fiber laser oscillator has a linear cavity. A semiconductor saturable absorber mirror (SESAM) is disposed at one end of the linear cavity. A polarization-maintaining gain fiber is operatively associated with the SESAM in the linear cavity, the gain fiber having normal dispersion. A polarization-maintaining undoped fiber is operatively associated with the SESAM in the linear cavity, the undoped fiber having anomalous dispersion. An output coupler is configured to generate laser light output from the linear cavity.

BACKGROUND

Mode-locked fiber lasers have been of considerable interest for the last 25 years based on their usefulness in applications such as optical frequency combs, laser machining, ultrafast microscopy, and various other research areas. Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12s) or femtoseconds (10−15s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example in modern refractive surgery. The basis of the technique is to induce a fixed-phase relationship between the longitudinal modes of the lasers resonant cavity. Constructive interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be ‘phase-locked’ or ‘mode-locked’.

Pulsed mode-locked lasers may be viewed in the frequency domain as a series of continuous lasers spaced out by the pulse repetition rate. When both the offset and the spacing between these discrete continuous lasers is controlled, the mode-locked laser is called an optical frequency comb. For applications in optical frequency combs, recent designs from NIST have demonstrated a passively mode-locked laser constructed of all polarization-maintaining (PM) optical fiber and fiber components in a simple linear cavity.

A PM optical fiber is highly birefringent and is capable of guiding maintaining light in one of two orthogonal linear polarization states. Because of this property, the polarization state of the circulating light is preserved in the laser cavity, resulting in an oscillator that is very robust to environmental disturbances. However, oscillators constructed from fiber with anomalous dispersion are not ideal for all frequency comb applications. Because of the large net anomalous dispersion, the pulse widths for this design are larger, when compared with other ultrafast oscillators that are dispersion managed. Therefore, this design suffers from increased phase noise in optical frequency comb applications.

There do exist dispersion-managed PM cavity designs that reduce overall phase noise of the oscillators, but these either contain non-PM components, free-space components, or individual components that are used to lower the net dispersion.

DETAILED DESCRIPTION

Passive mode-locking is achieved by introducing a loss mechanism that promotes pulsed operation, i.e. high peak power, over continuous wave operation (low peak power). Passive mode-locking can be achieved using a saturable absorber mirror (SAM) configured to have increasing reflectivity as the incident pulse energy is increased.

Another important aspect to achieving mode-locking is dispersion management within the laser cavity. Dispersion is introduced when laser light passes through intra-cavity materials. Different optical materials have a characteristic dispersion per unit length, β2which is defined as the second derivative of the refractive index n with respect to the optical frequency ω, as expressed by Equation 1 (EQN 1).

When β2is negative (positive), the optical material is referred to as having anomalous (normal) dispersion. If the round-trip dispersion in the laser cavity is too high, the desired ultrashort pulses will not be attainable. Dispersion management involves introducing anomalous and normal dispersion optical elements into the laser cavity to keep the round-trip dispersion low.

An example passively mode-locked laser is disclosed herein as it includes a linear cavity, and at least a saturable absorber mirror disposed at one end of the linear cavity. The example passively mode-locked laser also includes polarization-maintaining gain fiber in the linear cavity, the gain fiber having a normal dispersion output coupler. The example passively mode-locked laser also includes a polarization-maintaining undoped fiber in the linear cavity, the undoped fiber having an anomalous dispersion output coupler.

Before continuing, it is noted that the examples described herein are provided for purposes of illustration, and are not intended to be limiting. Other devices and/or device configurations may be utilized to carry out the operations described herein.

The operations shown and described herein are provided to illustrate example implementations. It is noted that the operations are not limited to the ordering shown. Still other operations may also be implemented.

It is further noted that as used herein, the terms “includes” and “including” mean, but is not limited to, “includes” or “including” and “includes at least” or “including at least.” The term “based on” means “based on” and “based at least in part on.”

In addition, “polarization maintaining fiber” as used herein means, but is not limited to, a fiber that has a waveguided core that also has a birefringence that allows guidance of two orthogonal polarization states along its two principal polarization axes.

“Anomalous dispersion” as used herein means, but is not limited to, the second derivative of the refractive index with respect to optical frequency is less than zero

“Normal dispersion” as used herein means, but is not limited to, the second derivative of the refractive index with respect to optical frequency is greater than zero

“Mode-locking” as used herein means, but is not limited to, a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12s) or femtoseconds (10−15s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example in modern refractive surgery. The basis of the technique is to induce a fixed-phase relationship between the longitudinal modes of the laser's resonant cavity. Constructive Interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be ‘phase-locked’ or ‘mode-locked’.

“Repetition rate” as used herein means, but is not limited to, the frequency with which pulses exit the fiber laser oscillator.

“Second order dispersion” as used herein means, but is not limited to, the 2nd derivative of the refractive index of a material or waveguide with respect to either frequency or wavelength. The preferred units used here are for

are in

“Comb-locker” as used herein means, but is not limited to, a device consisting of a fiber collimator, a cat's eye lens, and a semiconductor saturable absorber mirror (SESAM).

FIG. 1is an example all polarization-maintaining, passively mode-locked linear fiber laser oscillator10.FIG. 2is a schematic diagram of the example fiber laser oscillator10ofFIG. 1. An example all polarization-maintaining, passively mode-locked linear fiber laser oscillator may include a comb-locker12and an output coupler14. A polarization-maintaining gain fiber16is operatively associated with the comb locker12and the output coupler14. In an example, the gain fiber has normal dispersion. A polarization-maintaining undoped fiber18is operatively associated with the comb locker12and the output coupler14. In an example, the undoped fiber has anomalous dispersion.

Fusion splices20aand20bare indicated by X's inFIG. 2. In an example, splices may be reduced or eliminating by having the fiber components manufactured with the appropriate fiber. For example, the WDM could be constructed so that it uses only the passive fiber on the SESAM side of the cavity and the gain fiber on the output coupler side of the cavity. In another example, the fiber attachments to the SESAM and the output coupler may be manufactured as part of the WDM assembly.

In an example, the optical spectrum and temporal pulse width can be adjusted to lower phase noise. In addition, the net cavity dispersion can be tailored or specified by adjusting the relative lengths of the passive fiber18having anomalous dispersion and rare-earth doped fiber16having normal dispersion.

The tuning of the cavity dispersion allows the user to select the mode-locking regime that the laser10will operate in. For example, with a net anomalous dispersion cavity, the laser10may modelock in the soliton regime. In the soliton regime, a balance between the net-anomalous cavity dispersion and pulse non-linearity yields stable pulse propagation with a near transform limited pulse duration.

To further fine tune the performance of the optical cavity of the laser10, dispersion management can be applied to the all-PM fiber laser configuration by minimizing the net roundtrip anomalous dispersion. This leads to shorter optical pulses and lower phase noise.

One can also construct net normal roundtrip dispersion all PM laser oscillators. In this case, the oscillator may mode lock in the similariton regime. The similarity regime is favorable for generating high energy pulse and pulse amplification in a normal dispersion gain fiber16, and gives rise to the formation of a parabolic pulse that undergoes compression in anomalous passive fiber18.

To build mode locked lasers10with these varying mode-locking regimes, only the relative lengths of the normal and anomalous dispersion need to be adjusted. The disposition of optical elements (e.g., the saturable absorber and the output coupler) relative to the constituent optical fibers (gain fiber16and passive fiber18) can be optimized in order to optimize the output pulse length. To calculate the net cavity dispersion, the 2nd order dispersion of fibers used in the cavity can be multiplied by each subsequent length of fiber used and then summed together, as expressed by Equation 2 (EQN 2).

In Equation 2, the subscripts a and b refer to the fibers. To model the temporal and chromatic optical pulse evolution within the cavity, numerically solve the Nonlinear Schrödinger Equation (NLSE), which includes the gain, dispersion, and loss of the fibers.

From this numerical evaluation of the round-trip pulse evolution, two techniques are derived. First, the disposition of optical elements (such as the output coupler and the saturable absorber) relative to the fibers (gain fiber16and passive fiber18) can be set to minimize optical pulse width. Second, the disposition of optical elements (such as the saturable absorber) relative to the other components of the cavity can be selected such that intensity damage thresholds are not exceeded.

It is noted that the dispersion management technique described above may be realized with more than two types of fiber by generalizing Equation 2 and the numerical model to include more than two fibers.

Examples of an all polarization-maintaining, passively mode-locked linear fiber laser oscillator which implement these aspects are described below. One example is an “output coupler” oscillator configuration shown inFIGS. 3 and 4. Another example is a “mid-tap” oscillator configuration shown inFIG. 5. Still other examples which may implement the teachings described herein are also contemplated, as will be readily appreciated by those having ordinary skill in the art after becoming familiar with the teachings herein.

FIG. 3is a schematic diagram of an example “output coupler” configuration100of an example all polarization-maintaining, passively mode-locked linear fiber laser oscillator.FIG. 4is a schematic diagram of an example “WDM” configuration100′ of an example all polarization-maintaining, passively mode-locked linear fiber laser oscillator.

An example of the “output coupler” fiber laser oscillator is built with all polarization-maintaining (PM) fiber elements116and118arranged in a linear fashion between a SAM112and a partially-reflecting output coupler114. On one end of the laser cavity is a SAM112that enables the mode-locking within the laser cavity101. Typically, the SAM112is of a semiconductor design (which are referred to herein as a SESAM), but other saturable absorbers are also possible.

In an example, the SAM may be butt-coupled to the fiber end. For optical frequency comb applications that require precise tuning of the repetition rate, this component may also provide a method to change the length of the linear cavity101. For example, a fiber-coupled “comb-locker” module may include a fiber collimator, a cat's eye lens, and a SESAM mounted to a piezoelectric transducer (PZT). It is noted that there are other techniques to tune the length of the cavity101, such as fiber stretchers and thermal control.

In an example, the output coupler114of the laser oscillator is a thin-film mirror placed at the other end of the cavity opposite the SAM112. This may be a partial-reflection mirror butt-coupled to the fiber end, or the end of the fiber could be coated with a thin-film partial reflector.

In an example, a pump light120can be coupled into the cavity101by an output coupler114that is transmissive at the pump wavelength.

In an example, a wavelength division multiplexer (WDM)122can be incorporated inside of the cavity101. Though it can introduce extra loss in the laser cavity, the WDM122has certain benefits. For example, the WDM122and gain fiber116can be arranged such that the pump light120does not impinge on the SAM112, thereby avoiding potential damage to the SAM. Also for example, the output coupler114may reflect the pump light120so as to double pass the gain fiber116. This recycling of un-absorbed pump light increases the overall power efficiency of the laser.

In an example, the fiber is all PM and includes both anomalous dispersion passive fiber118(such as PM1550 fiber) and normal dispersion gain fiber116(such as small-core, erbium-doped fiber). In an example, the fiber lengths of the two types of fibers116and118are adjusted to tune the net cavity dispersion. There are constraints associated with both the minimum and maximum lengths of gain fiber116used inside of the oscillator. That is, too little gain fiber116, and the laser oscillator will not mode-lock. But too much gain fiber116, and the laser operation may not be stable. This allows designs of the laser cavity101in different mode-locking regimes that have different performance characteristics.

In an example, parts or all of the anomalous fiber118and normal gain fiber116can be changed to adjust the pulse width of the laser oscillator output. For example, the locations of the normal gain fiber116and anomalous passive fiber118sections may be swapped.

For optical frequency comb applications minimizing losses in the oscillator cavity101is critical to lowering the phase noise of the comb modes. As such, the device lends itself to low loss.

It is noted that each fiber splice used to connect fibers and fiber-coupled components introduces losses at the level of 0.1 dB to 10 dB, dependent on the fiber types being joined. An example relates to producing ultrafast pulses in the 1500-1600 nm wavelength regime by using a normal dispersion erbium-doped gain fiber. Because of the large infrastructure of robust fiber and fiber-coupled components available because of the optical telecommunications market, it may be desirable to design oscillators for this wavelength range. As noted above, splices may be reduced or completely eliminated.

FIG. 5is a schematic diagram of an example “mid-tap” configuration200of an example all polarization-maintaining, passively mode-locked linear fiber laser oscillator. The “mid-tap” fiber laser oscillator may be built with all polarization maintaining fiber elements and arranged into a linear cavity201. In this example, one end of the laser cavity201includes a comb-locker212. The comb-locker212includes a fiber collimator, a cat's eye lens, and a SESAM mirror. The other end of the oscillator cavity201includes a high reflector or end mirrors230.

The output coupler of the laser oscillator is a tap214or fiber splitter that is located somewhere between the two ends of the cavity201. Pump light220is coupled into the cavity201by a WDM222inside of the cavity201. The output coupler fiber splitter and the pump WDM222can be integrated into a single device within the cavity201. The fiber used in this oscillator is all polarization maintaining and consists of both anomalous dispersion passive fiber218(such as PM1550 fiber) and normal dispersion gain fiber216(such as small-core, erbium doped fiber).

In an example, the fiber lengths of the two types of fibers216and218may be adjusted to tune the net cavity dispersion. There are constraints associated with both the minimum and maximum lengths of gain fiber216used inside of the oscillator. Too little gain fiber216, and the laser oscillator will not mode-lock. Too much gain fiber216, and the laser operation may not be stable. This allows configurations of the laser cavity201in different mode-locking regimes that have different performance characteristics. For example, parts or all of the anomalous fiber218and normal gain fiber216may be arranged to adjust the pulse width of the laser oscillator output.

It is noted that the examples shown and described are provided for purposes of illustration and are not intended to be limiting. Still other examples are also contemplated.