Apr 3, 2007 - [4], and improve the capacity of communication channels [5]. ... can permit superluminal communication using entanglement; thus, relativity is ...
Tools for quantum optics: Pulsed polarization-maintaining Er-doped fiber laser and spatial mode manipulation in spontaneous parametric downconversion by
Dheera Venkatraman Submitted to the Department of Electrical Engineering and Computer Science in Partial Fulfillment of the Requirements for the Degree of Master of Engineering in Electrical Engineering and Computer Science at the Massachusetts Institute of Technology May 2007
@ 2007 Massachusetts Institute of Technology All rights reserved.
Signature of Author ......................... .... ....................... Department of Electrical Engineering and Computer Science May 25, 2007
Certified by ..............
.................... Franco N. C. Wong Thesis Supervisor
Accepted by................•
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'••/
,
..
Arthur C. Smitlh Professor of Electrical Engineering Chairman, Department Committee on Graduate Theses OFTECHNOLOGy
OCT 0 3 2007 LIBRARIES
ARCHIVES
Tools for quantum optics: Pulsed polarization-maintaining Er-doped fiber laser and spatial mode manipulation in spontaneous parametric downconversion by
Dheera Venkatraman Submitted to the Department of Electrical Engineering and Computer Science in Partial Fulfillment of the Requirements for the Degree of Master of Engineering in Electrical Engineering and Computer Science May 2007 S ?co71j
ABSTRACT Two separate projects were undertaken to improve technology for entangled photon sources, useful for quantum optics. In one project, a pulsed, mode-locked erbium-doped fiber laser, designed to be used as a seed laser for a 390 nm source, was built using polarization-maintaining components to address polarization drift. The fiber laser operated at a center wavelength of 1560.0 nm with an output power of 1 to 2.5 mW, and mode-locked with a repetition rate of 31.1 MHz. The laser also exhibited bandwidth tunability from 0.045 to 0.095 nm, as a function of the input pump power. A commercial 5 W erbium-doped fiber amplifier and a second harmonic generation crystal were used to obtain pulses at 780 nm with an average power of 3 W. The next second harmonic generation stage, for generating the desired 390 nm output, remains to be built. In the second project, we tried to optimize the coupling efficiency of light generated from spontaneous parametric downconversion (SPDC) into single-mode optical fibers, which are useful for transporting entangled photons. Using a setup with a tunable 532 nm pump waist in a nonlinear crystal, we achieved an effective coupling efficiency of 48.8% of the 797 nm signal light into a single-mode fiber, higher than previously obtained in the laboratory. Efficient single-mode operation of SPDC would enable the construction of a high-flux fiber-coupled source of nondegenerate entangled photons at 797 nm and 1600 nm. Thesis supervisor: Franco N. C. Wong Title: Senior research scientist, Research Laboratory of Electronics at MIT
Contents 1 Introduction 1.1
Motivation .....................................
1.2
Organization
1.3
Basics of entangled states and their applications .
................................... ...............
2 Nonlinear Optics 2.1
Basic theory of nonlinear optics . . .
........................
2.2
Nonlinear optical processes .
2.3
Nonlinear crystals and quasi-phase-matching techinque
...............
3 Pulsed polarization-maintaining erbium-doped fiber laser 3.1
Motivation .
.................
. . . . . . . . . . . . . . .. . . .
13
3.2
Overall design ................
. . . . . . . . . . . . . . .. . . .
14
3.3
Seed laser design
. . . . . . . . . . . . . .. . . .
15
3.4
Use of polarization-maintaining fiber . . . . . . . . . . . . . . . . . . . . . .
17
3.5
Construction of the source . . . . . . ...
18
..............
... . ...... .........
. ..... . . . . . . . . .. . . . 19
3.5.1
Pump diode set-up . . . . . . . ..
3.5.2
Gain fiber evaluation . . . . . . ..
. . . . . .
. . . . . . . 20
3.5.3
Continuous-wave laser construction
. . . . . . .
. . . . . . 21
3.5.4
From cw laser to pulsed seed laser .
. . . . .
3.5.5
Adjustment of the repetition rate .
.
. . . . .
. 22
3.6
3.5.6
Characterization of the seed laser . ..................
3.5.7
Testing the Erbium-doped fiber amplifier . ...............
3.5.8
Construction of the second harmonic generation stages .......
Summary
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. .. ....
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.
27
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.
30
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31
4 Spatial mode manipulation in spontaneous parametric downconversion 4.1
Mode-matching and fiber optics . ..................
4.2
Optimizing the single-mode operation of SPDC
4.3
Experimental setup for optimizing coupling efficiency . ............
4.4
Optimization procedure
4.5
Coupling efficiency results . . . ...
4.6
Summary
...................
. .....
...
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42
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33 36
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33
. ...............
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26
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...
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44 .
45 46
5 Conclusions
47
6
49
Acknowledgements
List of Figures 1
Overall block diagram of the source ...............
. . . . . . . . . 15
2
Schematic of the seed laser.
. .... ....
3
Comparison of standard and polarization-maintaining fibers
. . . . . . . . . 17
4
Pulsed regim es
. . . . . . . . . 24
5
Spectrum of the seed laser ...................
6
Sample of bandwidth dependency on pump current . . . . . . . . . . . . . . 28
7
EDFA output power as a function of EDFA pump current
. . . . . . . . . 29
8
EDFA output spectrum .....................
. . . . . . . . . 30
9
SHG output power as a function of SHG input power . . . . . . . . . . . . . 31
10
Mode-matching example ....................
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16
. . . . . . . . . 27
. . . . . . . . . 35
1
Introduction
The field of quantum optics opens up new possibilities in how we may think about using light. Taking advantage of nonclassical states of light and quantum optical devices has led to potential innovations that implement quantum key distribution systems [1] and quantum computing [2], attempt to increase imaging resolution [3], provide higher quality lithography [4], and improve the capacity of communication channels [5]. The cornerstone of most of these quantum optical designs, however, is a source of entangled states-a quantum state of two or more particles-whose measurement outcomes are each probabilistic but correlated beyond what classical physics allows [6]. While preliminary polarization-entangled sources have been produced and used in various devices, much research remains to be done in making these sources suitable for practical implementations. Some of the most lacking aspects of current entangled sources include extremely low fluxes, inability to efficiently couple into optical fibers, lack of portability, and high cost. Thus, we focus our research on developing polarization-entangled sources and their components that address some of these issues.
1.1
Motivation
This research is broken down into two sub-projects, each of which is an essential component for efficiently generating high-quality polarization-entangled photon states. First, we wish to construct a compact 390 nm pulsed laser source (-100 ps pulses at a repetition rate of 31.1 MHz) from polarization-maintainng components, that is suitable for use in quantum optics experiments including the generation of entangled pairs at 780 nm. Second, we aim
to create a high-flux nondegenerate fiber-coupled polarization-entangled pair source using a phase-stable Sagnac geometry [7, 8], and in this project, investigate the efficiency of coupling down-converted light into single-mode optical fibers.
1.2
Organization
To understand the goals of this research, we complete this introduction by presenting a basic overview of entangled states, assuming a basic knowledge of quantum mechanics. A brief background in nonlinear optics, the central concept around which these projects are based, is then provided. We then discuss the pulsed fiber laser and the non-degenerate high-flux source in two separate sections, and then conclude with a discussion of future work.
1.3
Basics of entangled states and their applications
As mentioned earlier, central to many quantum optical devices is the notion of an entangled state. An entangled state is a joint quantum state between two or more particles which cannot be expressed as a tensor product of individual particle pure states. In typical cases, the measurement outcomes in a particular basis are highly correlated between the two particles regardless of spatial separation. This correlation is greater than any correlation that can classically exist. The following illustrates a specific quantum feature of entangled states. If particle A is
in state (10) + 11))/xi2 and particle B is in state (10) + I1))/-2, their measurement outcomes are probabilistic and independent of each other (that is, there is an equal chance of seeing
a 0 or 1 for A and an equal chance of seeing a 0 or 1 for B that is independent of A's measurement), and these two particles are not entangled. In such a case, the joint state of the two particles is simply their tensor product state:
(10) + I1>))/
(10) + 11))/v' = (10) I0)+ I0)11) + I1)I0)+ 11) 11))/2.
(1)
It is evident in this product basis that there is an equal chance of measuring the joint state
to be 10) 10) or 10) 11) or 11) 10) or I1)I1), as one would expect from considering the situation in terms of classical probability. However, if A and B are entangled, the joint state cannot be written as a product of two individual states. An example of an entangled state
is (10) 10) + I1)I1))/IV. If both particles are each measured, it is evident that both must measure to be 0 or both must measure to be 1, since there is zero probability amplitude for the possibilities 10) 1) and 11) 10). Moreover, even if the state is mesaured in a different
basis set, such as (10) ± I1))/v. instead of 10) and I1), nonclassical correlation is still obtained. The entangled state can be distinguished from a mixture of unentangled states by measurements in two incompatible bases (a Bell state measurement is one example of such an entanglement test). The nonclassical correlation will hold true even if the particles are separated by large distances before being measured; at first glance this may seem to imply action at a distance that violates relativity; however, this does not provide a gateway to superluminal communication of information [9], since the two sides have no way to influence the measurement of the state at the other side. No methods of standard quantum mechanics can permit superluminal communication using entanglement; thus, relativity is not violated
with it. The concept of entanglement [10, 11] is immensely useful with the aid of classical communication. Entangled states can be used to create secure key distribution systems [1], to perform linear optics quantum computing [2], transport quantum states from one system to another [12], potentially increase channel capacities in communications lines [5], and image at higher resolutions than possible with classical light [3], plus other proposed applications. Given that such a vast set of potential applications exist for entanglement, we now focus our attention on the problem of generating polarization-entangled photons in the laboratory. The most common method to produce entangled photon pairs is by means of spontaneous parametric down conversion (SPDC), a nonlinear optical process that converts single photons of a higher frequency into two lower-frequency photons [13] which can be entangled [14]. To understand this, we now present a brief background in nonlinear optics.
2
Nonlinear Optics
Nonlinear optics refers to a number of phenomena in a medium in which its polarization responds nonlinearly to the electric field of the incident light. Typcially, new frequencies of light are generated as a result of this, with greatly varying degrees of efficiency depending on the type of process and the medium's nonlinearity. Nonlinear effects are highly materialdependent, and are useful for a wide variety of applications, including in our case, the generation of polarization-entangled photon states.
2.1
Basic theory of nonlinear optics
In conventional linear optics, the medium's induced polarization vector is linearly proportional to the electric field strength of the incident wave as described by the relation
P = cxE
(2)
where E is the permittivity of the medium and X is the susceptibility of the medium. In nonlinear optics, however, the dependence on the electric field E is not necessarily linear. Typical nonlinear dependence can be generalized to a power series, described by
P = EX(1)E' + eX(2)+ 2 +
X
+ ...
(3)
where X(n ) are the n-th order susceptibilities. Many crystal materials, such as lithium niobate, have a large X(2) coefficient, thus, we take a particular interest in them for use in nonlinear optics experiments. Nonlinear optical devices take advantage of the higher order terms above to create input-output relations that do not necessarily preserve frequency composition, and are responsible for a wide variety of applications. In this research, nonlinear optical devices play a central role in the creation of entangled photon sources, both in desired forms, such as the nonlinear process for creating outputs at the preferred frequencies, as well as undesired forms, such as unwanted nonlinear effects that generate background light and noise. We now present a brief overview of some important processes that result from the nonlinear relation above.
2.2
Nonlinear optical processes
A number of nonlinear optical processes will be relevant to the experiments in this thesis. In general, nonlinear optical processes permit a change in the frequency composition of light. Most commonly described are perhaps the frequency-mixing processes, which include, among others: * Second Harmonic Generation (SHG), in which the input frequency is doubled (w 2w). * Sum Frequency Generation (SFG), a more general case than SHG in which two light frequencies are summed (wl, w2 -- w3 = W1 + w2 ). * Difference Frequency Generation(DFG), in which the output frequency is the difference between two input frequencies (W1,i
3
-
w2 = w3 - W1)
* Spontaneous Parametric Down Conversion (SPDC), of particular interest to us in generating entangled photons. SPDC causes an input photon to split into two photons whose frequencies sum to the input frequency. It is "stimulated" by vacuum fluctuations and converts only a small fraction of the input pump light (w3 --* w, w2 with w1 +w 2 = W3).
There are numerous other common nonlinear processes besides the frequency mixing processes. The Kerr effect, for instance, is of interest to us in that it is unwanted in our experiments. In the Kerr effect, the refractive index of a material responds to the electric field and the change is proportional to the square of the electric field. The Kerr effect is responsible 10
for self phase modulation (SPM) [15], a X(3)-order effect most prominent when ultrashort pulses travel through media such as single-mode optical fibers [16].
2.3
Nonlinear crystals and quasi-phase-matching techinque
Lithium niobate (LiNbO 3 ) is a particularly useful crystal for optical experiments. It has electro-optical, piezoelectric, nonlinear, and birefringent properties, among others, but we employ it for its large X(2) coefficient, making it useful for second-order three-wave mixing processes, and in particular, SPDC. Efficient frequency mixing can occur only when the nonlinear process is phased matched at the three wavelengths. For example, SPDC will only occur in a crystal in cases where the relation
S- - = 0
(4)
is satisfied (where kp is the pump k-vector, and k, and ki are the k-vectors of the so-called signal and idler frequencies generated by SPDC). In many cases, it is not often possible to precisely tune the input frequency, and in many cases, some tunability of the output frequency combination is desired. This can be done by employing quasi-phase matching (QPM). QPM, first proposed by Armstrong et. al. [17] and further explained by Byer, et. al. [18], employs a periodically-poled crystal, such as periodically-poled lithium niobate (PPLN) or periodically-poled potassium titanyl phosphate (PPKTP), where the crystal is composed of a large number of domains, evenly-spaced along the direction of light propagation, alternating in ferroelectric domain orientation. The
flipping of alternate domains is accomplished by applying a large electric field near its breakdown value at alternate spacing intervals. The resulting flips in the domain polarizability enables efficient nonlinear mixing that is about 40% of a perfectly phase-matched interaction. More importantly, this technique permits the use of the largest nonlinear coefficient (d33 ) of LiNbO 3 , which is not available for birefringently phase-matched interactions, resulting in an overall efficiency that is significantly higher than in the non-QPM case. In addition, the phase-matching condition required for quasi-phase matching is dependent on the poling period A:
kpkski
27r
A
(5)
By poling a crystal with a user-defined period A and fine-tuning the period by placing the crystal in a temperature-controlled oven, one can phase-match at any desirable operating wavelengths over the crystal's transparency window, making QPM a powerful tool in nonlinear optics.
3
Pulsed polarization-maintaining erbium-doped fiber laser
Our goal is to construct a pulsed ultraviolet source at 390 nm, intended for use as a pump for entangled photon generation at 780 nm. Our pulsed source is composed of a low-power pulsed seed laser at 1560 nm, an erbium-doped fiber amplifier, and two stages of second harmonic generation (SHG) using nonlinear crystals that convert the infrared pulses to the desired 390 nm wavelength. In this section, we describe the motivation for this source, our overall setup, the mechanisms for generating pulses in lasers, how we plan to build the seed laser source, and the basic design principle. We will discuss the laser characterization and experimental results.
3.1
Motivation
The 390 nm source we plan to build is a pulsed, mode-locked, low-power seed laser at 1560 nm which is then amplified and passed through two stages of second harmonic generation to obtain the desired 390 nm. A previous such seed laser was built in our laboratory by Onur Kuzucu [19], using standard non-polarization maintaining (non-PM) single-mode fibers and components. This yielded an uncertain polarization at the output of the system, which had to be corrected using a triple paddle system before feeding into an amplifier and second harmonic stages. The input to the PM amplifier requires a fixed, well-defined linear polarization from the seed laser. Similarly the two SHG stages also require a fixed linearly polarized input. For the non-PM laser, the polarization may drift over long periods of time or under 13
perturbations. Given these issues, we primarily concentrate our research on creating a PM version of this seed laser, using mostly PM fibers and components to ensure that the output polarization is well-defined and stable. Following this construction, we aim to characterize the seed laser and incorporate it into the amplification and SHG stages to achieve a 390 nm pulsed source that is compact, portable, and reliable.
3.2
Overall design
The UV source is composed of several stages that were built in sequence, as shown in the block diagram in Fig. 1. A 980 nm fiber-coupled laser diode pumps a fiber-based seed laser, which operates in a mode-locking regime to generate short pulses (v100 ps) at a repetition rate of 31.1 MHz at 1560.0 nm. The output of the seed laser is fed into a commerciallyobtained PM erbium-doped fiber amplifier (EDFA), which amplifies the pulsed output to an average of approixmately 5 W. The output of the EDFA, still at 1560.0 nm but amplified in power, is output-coupled to a collimator, and, in free space, fed through two stages of second harmonic generation (SHG) using nonlinear crystals, resulting in the final desired 390 nm wavelength. Although an earlier version of this source was built by Onur Kuzucu at MIT [19], in this research, we investigate the use of polarization-maintaining fibers and components in the seed laser to improve its stability and characteristics, and aim to produce a more compact version of the entire source. We now turn to the problem of designing the seed laser, the main component this source and this research.
Pump
Er-doped pulsed laser """"
980nm
1560nm
EDFA
"""t 1560nm
HG
390nm SHG ..... 780nm
Figure 1: Overall block diagram of the source.
3.3
Seed laser design
While pulsed lasers can be built in free-space, using a fiber medium as the laser cavity for the seed laser provides a number of advantages, including durability, resistance to vibrations, smaller physical footprint, and absence of intricate alignment requirements. Fibers can be doped, and all the necessary components to pump a fiber gain medium and output-couple light are available commercially. Our seed laser design, shown in Fig. 2, is composed of several components arranged in a ring geometry. A pump diode at 980 nm, suitable for pumping erbium, is coupled into the ring using a wavelength division multiplexer (WDM). The 980 nm light is pumped counter to the intended direction of lasing. An optimized length of Erdoped fiber is spliced to the WDM and serves as the gain medium for the laser cavity. Two circulators are spliced into the cavity, each of which permits light to flow directionally only from its labelled port 1 to port 2, and port 2 to port 3. These circulators permit reflective components to be included in the cavity. Attached to port 2 of one circulator is a Bragg grating at 1560.0 nm which fixes the laser wavelength; to the other circulator is a saturable absorber setup which mode-locks the laser and preferentially amplifies high-intensity pulses, causing the laser to output short pulses. The saturable absorber preferentially amplifies highintensity pulses in the cavity, and with proper alignment, can be used to achieve a modelocking state, in which the cavity modes have a fixed phase relationship with each other,
constructively interfering to yield short and high-intensity pulses. The final component in the laser cavity is an output coupler, which diverts 10 percent of the circulating power to the laser output at each round trip. The total length of the laser cavity will be adjusted to permit a 31.1 MHz pulse repetition rate, and the desired average output power is 1 to 5 mW. While a very similar seed laser was previously built, our objective is to build the entire seed laser out of polarization-maintaining components, to alleviate polarization instability issues, which are undesirable when connecting the seed laser to the amplifier in the next stage of the overall design.
01
mator ieric f=3mm
Figure 2: Schematic of the seed laser.
3.4
Use of polarization-maintaining fiber
In the overall design, it is important that a fixed and well-defined polarization be maintained at the input of a polarization-maintaining EDFA and also for second harmonic generation. As mentioned before, another student constructed a similar but non-PM source; however, this source required polarization corrections at its output and is susceptible to polarization drifts over time. This instability is a result of a lack of polarization selectivity in the construction of the laser. However, in this work, we improve upon the design by building it almost entirely out of polarization-maintaining components (with the exception of the Bragg grating, which was not commercially available in a PM version at the time). Polarization maintaining fibers contain embedded rods that induce mechanical stresses in the fiber, which in turn induces optical birefringence in the material. The difference in index allows light of a fixed polarization along one axis to more easily remain polarized in that axis. Since the fibers now have a specific orientation, it is important to note that they cannot be spliced using a standard splicer; a special polarization-maintaining splicer (such as those manufactured by Ericcson or Fujikura) must be used which inspects both cleaved fiber ends for their stress rod orientation and aligns the axes of birefringence before performing the splice. Cladding Core
e
Stress rods
Standard SM fiber
Panda PM fiber
Figure 3: Comparison of standard and polarization-maintaining fibers. Stress rods in the cladding of polarization-maintaining fibers maintain the polarization orientation.
3.5
Construction of the source
Since the polarization-maintaining EDFA is a commercial product and the second harmonic generation stages are relatively standard configurations, we omit a detailed description of their design. We now consider the process of building the entire UV source, which will be accomplished in several stages. First, we set up the pump diode and test its characteristics. Second, we test various commercial Er-doped gain fibers and select one that is suitable for our purposes. We then splice together a continuous-wave (cw) laser by including only the pump, WDM, gain fiber, Bragg grating (using one circulator), and output coupler. After testing this setup, we break a splice to insert the second circulator attached to the saturable absorber setup to convert it into a mode-locked pulsed laser. After testing its characteristics and ensuring that it mode-locks reliably, we adjust the repetition rate of the pulses to the desired value of 31.1 MHz by cutting unnecessary segments of undoped fiber inside the laser cavity. This value of 31.1 MHz matches a similar but non-PM version of the seed source previously built by Onur Kuzucu. This completes the construction of the seed laser. If its characteristics are satisfactory at this point, the output is then connectorized and attached to an IPG Photonics erbium-doped fiber amplifier (EDFA) in an original equipment manufacturer (OEM) configuration that allows us to customize the setup. The EDFA is tested and tuned, after which the second harmonic generation stages can be built to complete the construction of the UV source. We now examine the actual execution of these individual construction steps in detail.
3.5.1
Pump diode set-up
Since we plan to operate at 1560.0 nm, we use erbium-doped fibers with optical pumping at 980 nm. We obtained a 980 nm, 500 mW JDS Uniphase diode (29-8052-500) with a built-in thermoelectric (TE) cooler, designed to fit into a 14-pin butterfly mount and with a bare fiber output. The diode was mounted onto a Thorlabs butterfly mount, designed to support both of the two commonly-used pin specifications. This diode had a Type 1 specification, which was acommodated simply by fitting the appropriate board supplied with the butterfly mount. A Thorlabs ITC 100 OEM driver was used to control the diode and TE cooler, and required adjustment of the PID controller to properly and quickly reach a set operating temperature throughout the experiment (approximately 16-18 0 C). As it did not come with a power supply and required +12 to +15, 0, and -12 to -15 volt inputs, we initially used known clean power sources such as two HP power units in series. We then tested a Lambda Electronics OEM switching power supply unit (SCD601515), which provides all 3 necessary inputs with a rated maximum of 1 percent peak-to-peak ripple. One important concern with using switching power supplies is that significant components of the high switching frequency (and even 60 Hz line frequency in some cases) may leak into the diode driving current and cause an unwanted modulation in the light output. However, testing this particular power unit and using a high-speed photodiode and oscilloscope to look for frequency artifacts showed no noticable oscillations or difference when compared to the use of a linear power unit. We believe that the input power filtering mechanism in the ITC 100 driver was good enough for us to use the switching supply, which is a much less expensive and compact power
solution. An acrylic housing was made to hold both the ITC 100 and the SCD601515 units and forced-air cooled with four fans.
3.5.2
Gain fiber evaluation
An ideal gain fiber for use in the seed laser would have a high gain over a short distance, allowing for maximum freedom in adjusting undoped fiber lengths to achieve the desired repetition rate. Three commercially-available erbium-doped polarization maintaining fibers were tested for their suitability: Nufern PM-ESF-7/125, Fibercore DHB1500, and CorActive EDF-Er-25-05-PM. Each fiber was spliced in turn to the output of a custom-made Senko polarization-maintaining wavelength-division multiplexer (WDM), with one input connected to the 980 nm pump diode and the other input connected to a 1560 nm source of known average power of 1.5 mW. For each gain fiber, we began with at least 3 meters spliced to the WDM, and ran the JDS Uniphase diode at a fixed current level of about 560 mA, providing approximately 250 mW of pump power. The output power at the loose end of the gain fiber (after proper cleaving) was measured to determine the gain provided by the fiber at 1560 nm. At excessively long lengths, the Er-doped fiber attenuated the output, and at excessively short lengths, the pump power barely depleted; we looked for an optimal length with the maximum gain. This optimal length was determined by continually monitoring the output power while progressively cutting back and cleaving the Er-doped fiber in small intervals until it reached a maximum and just began to drop again. It is helpful that Er-doped fibers, under pumping, fluoresce in visible green as well, providing a hint of the extent of the length of the fiber that the pump light was being absorbed.
We found that the lightly-doped Fibercore and medium-doped Nufern fibers required long lengths to provide significant gain, and as briefly mentioned before, this would make it difficult to control the repetition rate of the laser later since there would have to be much less undoped fiber in the cavity to maintain the same total cavity length, thus requiring tight splices to be made. The highly-doped CorActive fiber, however, provided excellent results, and within 1-2 meters of fiber provided a gain factor of approximately 20 to 30. We therefore chose to use the CorActive fiber as our gain fiber.
3.5.3
Continuous-wave laser construction
Next, a continuous-wave laser was built. This was accomplished by splicing a PM 90-10 output coupler (Lightel PMC-S-12-9010-1550-1-B-0) and PM circulator (AC Photonics PMC-11-5-1-10-1-0-C-0) to the gain fiber and WDM previously used for the gain fiber evaluation. We attached the Bragg grating to the middle port of the circulator and built the entire cw laser system without the circulator associated with the saturable absorber (which is used for passive mode-locking operation). While polarization-maintaining splicing was performed for the circulator and coupler, the Bragg grating used (Teraxion, custom-made, 1560.0 nm, 25 GHz bandwidth) was not etched on a polarization-maintaining fiber. As a result, the laser's peak wavelength and spectrum changed if the fiber grating was shifted or curled. To solve this, we removed most of the non-PM pigtail on the Bragg grating, re-spliced it, and then housed the entire length of the Bragg grating section, including the short length of non-PM fiber, in a thin 25 cm long brass tube. Since the laser was built on a 12 x 12 in aluminum breadboard, the brass tube was taped directly to the breadboard for added stability. This 21
kept the entire non-PM section and Bragg grating straight and greatly improved the stability of the output spectrum. The output power of the cw laser was approximately 15 mW, with the pump current still at 560 mA.
3.5.4
From cw laser to pulsed seed laser
A pulsed laser, such as the seed laser being constructed, periodically emits short pulses with high peak powers, as compared to the steady output of the commonly used cw laser. There are generally two common methods for generating such a pulse train: Q-switching and mode-locking. In a Q-switched pulsed laser, extremely high-intensity pulses are produced at a relatively low repetition rate (typically on the order of kilohertz). This is created by placing a variable attenuator, such as an (active) attenuator component, or a (passive) saturable absorber, inside a laser cavity, and configuring the system in a way such that the gain medium attains a high population inversion, possibly near saturation, before suddenly changing the quality factor of the cavity to output the large amount of stored energy in the gain medium. The repetition rate of a Q-switched pulsed laser is dependent largely on the configuration, attenuator, and input power, and not on the laser cavity round-trip time. A mode-locked pulsed laser, on the other hand, produces a much higher repetition rate (tens of megahertz is common) and much shorter pulses (which can be anywhere in the femtosecond to picosecond ranges). In a mode-locked laser, the longitudinal modes of the laser cavity are in phase and they constuctively interfere to yield a short pulse (with a pulse width that is inversely proportional to the mode-locking spectral width). For our system, the cavity length is much larger than the wavelength, and the mode spacing for the ring
configuration is given by Sf = c/L, where L is the round-trip cavity length. In a modelocked laser, the various modes of a laser cavity operate with a fixed phase relationship to each other, typically producing a pulse train, with the overall repetition of the pulses being at a frequency of c/L. Mode-locking can be accomplished actively or passively. Here, we pay particular attention to passively mode-locked lasers, which typically use a saturable absorber to partially attenuate low-intensity light while permitting high-intensity light under saturation. This can cause a fluctuation in the laser cavity to be preferentially amplified, leading to a train of pulses as the laser's output coupler takes out a fraction of each pulse while the gain medium amplifies it again at the next round-trip. After the laser is in modelocking operation, additional fluctuations will not be amplified in the ideal case, and only the existing pulse train will continue to be selected and trigger stimulated emission at each round-trip. Since we wish to build a passively mode-locked fiber laser, it is evident from the above that two potential issues could arise. One is that since the same technology of saturable absorbers is capable of producing both Q-switching and mode-locking operation, that a mis-configured cavity could accidentally enter a Q-switching state. Another potential problem is "double pulsing", in which two amplified pulses appear together at each repetition interval and are both amplified during each round trip. However, with careful alignment and tweaking of free-space components, it is usually possible to place the operation of the laser in the desired state. Fig. 4 illustrates typical oscilloscope traces of lasers operating in the two pulsed modes. With knowledge of these potential obstacles in achieving proper mode-locking, the cw
JI Q-switching
Double-pulsing
LJ. I I Properly mode-locked
Figure 4: Pulsed regimes: In the Q-switching scenario, very large pulse envelopes are observed, repeating on the order of kilohertz, with underlying repeating pulses at the intended repetition rate. In double pulsing, the laser is mode-locked but starts with two pulses simultaneously being amplified at each round trip. In a properly mode-locked scenario, consistent height pulses are observed at the intended repetition rate. laser cavity was cleaved apart at one point and a second circulator (AC Photonics PMC-1-15-1-10-1-0-C-0) was inserted in order to include a saturable absorber. The saturable absorber chip (Batop 1550-30-0) was fixed to a small aluminum disc and mounted on a 3-knob locking mirror mount. The 3 mm aspheric lens was mounted with an X-Y translating mount and roughly aligned to a 3 mm distance from the saturable absorber. The collimator was mounted using an 2-knob tilting mount approximately 2 cm from the aspheric lens, and spliced to the second circulator now in the cavity. At this point, lasing will only occur if the center port of the second circulator reflects light back into the cavity; that is, there exists a properlyaligned round trip path out of the collimator and coupled back into the same fiber. The alignment of the free space components was a challenge, but done with the help of using a high reflector (HR) mirror in place of the saturable absorber at first (thus creating a cw laser again), and then substituting the HR mirror for the saturable absorber and fine-tuning the alignment. Using a sensitive fiber-coupled power meter with the laser output also facilitated the alignment process, as minor changes in power often hinted towards the proper alignment
as the geometry of a component was tweaked. Once the saturable absorber was aligned properly, pulsing was easy to observe with a high-speed Thorlabs InGaAs photodiode, but as pointed out before, Q-switching situations occurred if the alignment was not correct for mode-locking. In some cases, a situation was seen where the laser could be aligned to mode-lock, but if it was powered down and powered up again, it entered into a Q-switching state with some high probability. However, with enough tweaking of the saturable absorber alignment and focusing, it was possible to find an alignment where the laser consistently entered a stable mode-locking state within 1 second of starting up. Initially, the seed laser operated at a repetition rate of approximately 13 MHz and an average output power of about 2 mW.
3.5.5
Adjustment of the repetition rate
To achieve the desired 31.1 MHz repetition rate of the design specifications, excess fiber was removed from the ring cavity. The excess fiber removal was a challenging process, because of PM components that required proper fiber orientation as well as tight splices that had to be made. Though we had initially used an Ericcson PM fusion splicer and another older splicer using a tungsten filament, the final re-splicing was accomplished with a much more reliable Fujikura PM splicer at Lincoln Laboratory. First, extra fiber lengths were removed from the middle port of the circulators, as cutting down on those lengths would have double the effect due to the round trip through those fibers. After that, further fiber lengths were removed between components, but the gain fiber was left intact. With careful estimation, we achieved a repetition rate of 31.15 ± 0.05 MHz. This frequency should also be fine-tunable
by adjusting the distance from the collimator to the aspheric lens and saturable absorber. This completed the construction of the polarization-maintaining seed laser.
3.5.6
Characterization of the seed laser
Using an optical spectrum analyzer, we characterized the seed laser and found it operating accordingly to what we had expected. The center wavelength was very stable over time and centered around 1559.88 nm. With the initial pump power settings, we observed an average output power of 2.2 mW and a bandwidth of 0.09 nm (FWHM); the spectrum is shown in Fig. 5. However, one interesting aspect, which was different from the non-PM seed source built earlier [19], was that the bandwidth was tunable by adjusting the pump power. We observed that a higher pump power supplied to the ring laser resulted not only in a higher average output power, but also an increase in bandwidth. Since there was a limited operating range of pump powers that would enable the ring laser to properly mode-lock, the bandwidth tunability was limited to approximately 0.045 nm to 0.095 nm with the initial settings. A graph of this dependence is shown in Fig. 6. However, the actual range of tunability was heavily dependent on the alignment of the saturable absorber, but it should generally be close to this range. Since the input specification of the EDFA allows for a range of average powers from 1 to 5 mW without significant changes in the output, the average output of the seed laser is essentially irrelevant as long as it is within this range, and the tunability can be a potentially useful feature. The polarization extinction ratio of the seed laser was measured using a rotatable wave plate, polarizing beam splitter, and power meter, and found to be at least 20 dB with the axis of polarization stably fixed along the slow axis of the fiber. 26
Given this analysis, we claim that the construction of a passively mode-locked polarizationmaintaining Er-doped fiber laser was successful and we accomplished the goal of achieving polarization stability while still maintaining the desired characteristics for the rest of the laser source. 2007 Apr 03 21:36 SPECTRAL WIDTH : THRESH LUL1 : 3.00dB K: 1.00 • : 0.090nm B:FIX /BLK THRESH LUL2 : 13.00dB MODE: 1 C: 1559.882nm C:FIX /BLK 6.37pW/D RES:0.01nm SENS:NORII HLD AUG: I SMPL:AUTO i
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Figure 5: Spectrum of the seed laser, measured by an optical spectrum analyzer, showing a bandwidth of 0.090 nm, driven with a pump current approximately 470 mA, and with a measured output power of 2.2 mW.
3.5.7
Testing the Erbium-doped fiber amplifier
The erbium-doped fiber amplifier (EDFA) was custom-made by IPG Photonics and accepted a fiber input from 1 mW to 5 mW. The output cable with a 14 pm core diameter designed to carry higher powers, was well-protected and factory-attached to a collimator. The maximum rated input current was -6 A, yielding an output power of -5 W. Testing of the EDFA was broken down into three subcategories: power output, polarization extinction and stability,
Sample of bandwidth dependency on pump current
E cc
I U-
ao Input current (mA)
Figure 6: Sample of the bandwidth dependency on input pump current with the initial construction. and spectral characteristics. First, the power output of the EDFA was tested by attaching the input to an Agilent cw laser module tuned to 1560 nm at a power of 2.2 mW, and the EDFA output power was plotted as a function of input current. The seed source was then connected and the same power measurement repeated, obtaining almost identical results in power output, as one would expect. The power dependency on EDFA current was nearly linear, and is shown in Fig. 7. Second, the polarization extinction of the EDFA was measured with the Agilent cw laser and seed laser as inputs. Contrary to our expectations, the polarization fluctuated rapidly, oscillating from one orientation to another on a time scale of 10-30 seconds with a poor extinction ratio. Finally, we checked the spectrum of the laser using a spectrum analyzer, and found that self-phase modulation (SPM) was taking place. SPM is a phenomenon caused by the nonlinear Kerr effect in the fiber medium, in which
CL CL
0. 0. 0 LL
0
LU
EDFA current (mA)
Figure 7: EDFA output power as a function of EDFA pump current. the refractive index of the material exhibits a small dependence on the optical power travelling in the medium, greatly impacting the spectra of high-intensity ultrashort pulses. Typically, and in our case, the spectrum begins to broaden and separate into two or more spectral peaks with increased SPM. Following this analysis, the EDFA was taken to IPG Photonics for two modifications. First, to correct the polarization instability of the EDFA output, a polarizer was inserted into the EDFA This ensured that all polarizations were kept along the slow axis of the fiber and the polarization fluctuation problem was corrected. Second, the length of the output fiber was shortened by 0.4 m (since such a long output fiber was not necessary for our setup) that reduced the amount of SPM as shown in Fig. 8.
2BBTApr 63 21:48 :
