Ultra-Stable Lasers
First Semester Report
Fall Semester 2011
- Full report –
by
Steven Dorlac
Andy Wiersma
DaxsamuelChitechi
Derrick Benallie
Prepared to partially fulfill the requirements for
ECE401/ECE402
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, Colorado 80523
Project advisor: Prof. Randy Bartels
ABSTRACT
As technology advances computers are working faster and faster. With faster speeds, more accurate time keeping methods are needed. Optical clocks are one method being researched to replace atomic clocks in their role for accurate time keeping. This experiment is designed to investigate and improve the use of stable optical combs that will replace the current atomic clocks with more accurate time keeping.
In this project we will build an ultra-stable external cavity diode laser, as well as analyzing electronics for temperature stabilization, low-noise diode drivers, and control electronics for feedback stabilization of temperature and laser frequency. The ultra-stable laser will improve the research on communications to remote synchronization. This semester we are focused mainly on the design and testing of the electronic components of the ultra-stable laser. The electronics will be composed of four different circuits, which are a low noise power supply circuit, thermal electronic control circuit, diode driver circuit, and a loop electronics circuit. For the spring semester of 2012 we are going to combine the four circuits with a laser design to create an ultra-stable laser for time keeping devices.
Through analyzing different laser setups we were able to narrow down which ones applied the best to optically based clocks. We decided to base out laser off of the designs made by JILA of CU Boulder. There are many parts of their design that we found were still not fully applicable to our desired function so this paper will focus on the parts that are the most relevant.
TABLE OF CONTENTS
Title……………………………………………………………………………………………..….i
Abstract………………………………………………………………………………………...….ii
Table of Contents…………………………………………………………………………………iii
I)Introduction………………………………………………………………………………..1
II)Optical Clockworks……………………………………………………………………….2
A)Optical Clock Intro………………………………………………………………..2
B)Doppler Broadening……………………………………………………………….3
C)Overview of Mode-locking and Frequency Combs……………………………….3
III)Laser Diodes………………………………………………………………………………4
A)Introduction………………………………………………………………………..5
B)Physics…………………………………………………………………………….5
C)Types………………………………………………………………………………7
i)Heterostructure Laser Diodes………………………………………..……………7
ii)Quantum Well Laser Diodes………………………………………………………8
iii)Distributed Feedback Lasers………………………………………………………8
iv)Vertical Cavity Surface-Emitting Laser Diode……………………………………9
v)External Cavity Diode Laser………………………………………………………9
D)Characteristics……………………………………………………………………10
i)Coherence……………………………………………………..…………………10
ii)Power…………………………………………………………….………………11
iii)Temperature Dependencies………………………………………………………11
IV)Mode Locking……………………………………………………………………………12
A)Mode-Locking……………………………………………………………………12
B)Types of mode-locking………………………………..…………………………13
i)Active……………………………………………………………………….……13
ii)Passive……………………………………………………………………………15
iii)Hybrid……………………………………………………………………………16
C)Applications of mode-locking a laser……………………………………………16
D)Issues with mode-locking…………………..……………………………………16
V)Frequency Combs……………………………………………..…………………………18
A)Introduction to Frequency Combs………………………….……………………18
B)Frequency Comb generation………………………………..……………………19
C)The Carrier-Envelope Offset………………………………..……………………20
i)Carrier-Envelope Offset Stabilization……………………………………………21
D)Noise in Frequency Combs………………………………………………………22
E)Applications……………………………………………………………...………23
VI)Fall 2011 Semester Plans/Accomplishments…….………………………………………24
A)Introduction………………………………………………………………………24
B)Current Controller Circuit…………………………..……………………………25
C)Thermoelectric Controller (TEC) circuit……………...…………………………28
D)Power Supply……………………………………………….……………………32
E)Control/Filter Loop Electronics…………………………….……………………36
VII)Spring 2012 Semester Plans……………………………………………...………………39
References……………………………………………………………………………..…………40
Bibliography……………………………………………………………………………..………42
Appendix A)Abbreviations……………………………………………………………...……A-1
Appendix B)Budget………………………………………………………………………..…B-1
LIST OF FIGURES
Figure 1-Schematic of Optical Clock………………………………………..……………………2 Figure 2- Laser Mode Structure……………………………………...……………………………2
Figure 3- Mode Locking Laser……………………………………………………………………3
Figure 4 –Dirac Function……………………………………………………….…………………4
Figure 5 – PN Junction……………………………………………………………………………5
Figure 6 Direct vs. Indirect Bandgap………………………………………………………...……6
Figure 7 – Heterostructure Diode…………………………………………………………………7
Figure 8 – Quantum Well Diode…………………………………………………………..………8
Figure 9 – VCSEL…………………………………………………………………………...……9
Figure 10 – ECDL Configurations……………………………………………………….………10
Figure 11: Generation of a pulse train from optical oscillations interfering with each other....…12
Figure 12: mode locked spectrum and a spectrum that is not properly locked……………..……13
Figure 13: Setupof an actively mode-locked laser ………………………………………..……14
Figure 14: Setup of a passively mode-locked laser………………………………………...……15
Figure 15: frequency comb of a mode-locked laser……………………………………...………18
Figure 16: Electric field of laser pulses with a 5fs duration and variable CEO phase……..……20
Figure 17: Principle of the common f-to-2f self-referencing scheme…………………...……….21
Figure 18: Feedback portion from Jila Current Controller Schematic………………….……….25
Figure 19: Output to Laser Diode from Jila Current Controller Schematic……………..………26
Figure 20: Current Line from Jila Current Controller Schematic………………………………..26
Figure 21: Supply to Current Line from Jila Current Controller Schematic…………….………27
Figure 22: JILA Temperature controller circuit…………………………………………...……..29
Figure 23: Low power, low cost 2.5V Reference……………………………..…………………30
Figure 24: Low power, low cost 2.5V Reference top view…………………………..…………30
Figure 25: Gain stage of the temperature controller…………………………………..…………31
Figure 26: Peltier cooling and heating stage………………………………………..……………32
Figure 27-Pulsed Power Supply………………………………………………….………………34
Figure 28-Typical Input vs. Output Voltages…………………………………..………………..35
Figure 29-DC to HV DC Converter Table (G03)……………………………….……………….35
Figure 30 – Controller Block Diagram.…………………………………………………………36
Figure 31 – JILA Controller…………………………………………..…………………………37
Figure 32 – JILA Controller………………………………………..……………………………38
1
- Introduction:
This experiment is designed to investigate and improve the use of stable optical combs that will replace the current atomic clocks with more accurate time keeping. In this project we will build ultra-stable external cavity diode laser, designing electronics for temperature stabilization, low-noise diode drivers, and control electronics for feedback stabilization of temperature and laser frequency. The ultra-stable laser will improve the research on communications to remote synchronization. For the fall semester we focused mainly on the design, build and testing of the electronic components of the ultra-stable laser. The electronics will be composed of four different circuits, which are a low noise power supply circuit, thermal electronic control circuit, diode driver circuit, and a loop electronics circuit. For the spring semester of 2012 we are going to combine the four circuits with a laser design to create an ultra-stable laser for time keeping devices.
- Optical Clockworks
- Optical Clock Intro:
An optical clock (figure 1) is a clock that is based on fundamental physics of an electron in an atom. When these atoms are subject to an electromagnetic field, they absorb energy and jump to a higher energy state. As these electrons re-emit their energy, they drop back down to lower states. If a feedback loop consists of electrons that continually fluctuate between two levels, you can construct an ultrafast and a precise subatomic pendulum. An optical clock can offer an extremely high frequency precision and stability which are kept in an optical trap. When you apply laser cooling, which is limiting the random motion of the dissipative light forces, you reduce the collisions and the temperature of these particles to suppress Doppler broadening.
- Doppler Broadening:
Doppler broadening is the broadening of spectral lines due to the Doppler Effect. This is caused by a disturbance of velocities in the atoms. The atoms of reemitting energies have different velocities that result in different Doppler shifts that cause line broadening. Doppler broadening can have severe constraints on spectroscopic measurements.
is the Doppler broadening, which depends on the frequency of the spectral line, the mass of the emitting particles, and their temperature. By reducing the temperature by laser cooling, it decreases atomic collisions; therefore you can have more precise experimental outcomes.
Optical Clockworks are frequency chains that contain complicated combination of many nonlinear stages. Each of these stages has some frequency that contains multiples of that same frequency. Not only are these frequency chains difficult to make, but we’re limited to certain isolated optical frequencies. With the use of frequency combs that are produced from mode-locked lasers it makes it simpler and more versatile to produce optical clockworks.
- Overview of Mode-locking and Frequency Combs:
A mode locked laser (figure 6) is a technique used in optics that produces pulses of light in a very short duration. This technique is based on an induced fixed phase between the modes of the laser’s resonant cavity. This is what makes the laser become mode-locked. When the laser is mode-locked, it produces a train of pulses that can be as short as a few femtoseconds. These lasers are able to produce frequency combs, which are determined by the pulse repetition frequency and the carrier envelope offset frequency. A certain integer multiple of the pulse repetition frequency and a beat note frequency can be measured and processed with fast electronics. The optical cavity of the laser determines the laser’s emission frequency. Basically by facing two flat plane mirrors toward each other, the light of the laser oscillates back and forth between the mirrors and produces a gain. The light begins to destructively and constructively interfere which creates modes between the mirrors. These are the longitudinal modes of the cavity. These modes have a narrow range of frequencies where it operates, which is the bandwidth. For the above figure, SA is saturable absorber mirror. This means that the reflectance of this mirror is 100% and that the incoming signal will be totally reflected. OC is a coupler mirror that has a reflectance less than 100%, so minimum amount of the signal will propagate through the mirror but the majority of the signal will be reflected back into the cavity.
When the mode lock lasers generate the optical pulses, as seen through the spectrum, you will see a series Dirac delta functions (figure 4). These functions are produced by the oscillations of the light reflecting back and forth between the mirrors of the cavity. The separation of these Dirac functions are separated by the time it takes the light to complete one full cycle.
- Laser Diodes
- Introduction:
Laser diodes possess many features that give them a wide range of applications from simple low power laser pointers to very high powered laser diode arrays found in laboratories and manufacturing. They are available in a wide arrange of wavelengths, power ratings and geometries. Additionally, they are compact and often possess a high electrical to optical efficiency lending themselves to applications with space and power restrictions. What follows is a brief description about the theory of operation behind laser diodes in addition to some of the more common types available.
- Physics:
Lasers Diodes form a subset of the diode family. Like all diodes, their simplest form is composed of two differently doped regions within the chip, a p-doped region and an n-doped region. The junction of these two regions creates what is known as a depletion region. This depletion region is characterized by an absence of charge carriers caused by the combination of “holes” in the p-region and electrons in the n-region. When an electrical potential is applied to the diode in the forward direction, electrons and holes are injected into the depletion region from the n and p regions respectively allowing current to flow. When the depletion region has both holes and electrons they can recombine which results in the release of energy either in the form of light (photons) or heat (phonons). A graphical representation of this is seen with a band-gap diagram pictured below.
The main difference between a standard diode and a light emitting diode (LED) lies in the alignment of the valance band and conduction band in the band-gap diagram. This alignment allows electrons to transition to the valence band directly releasing the energy as light (photons). In a normal diode the bands are misaligned causing the release of energy in the form of heat (phonons). Laser Diodes are a specialized form of an LED. The ends of a laser diode are polished or cleaved in such a way that the ends form an optical resonance cavity. Additionally, a laser diode has an increase in the amount of carriers present due to a modification of both current and geometry creating an abundance of electrons and holes with the potential to combine. With this increase in carrier population it now becomes possible for a passing photon released from an electron and hole combination to trigger another combination without being absorbed. This combined with the resonance cavity allow for the generation of more photons with matching phase. A portion of this coherent light is emitted from the diode chip since the ends are not completely reflective. This light is what comprises the laser produce by the laser diode.
- Types
The physics discussion above was limited to a single p-n junction laser diode due the need to simplify its explanation. This type of laser diode is known as a homojunction and is subject to limitations and inefficiency issues that are not present in more complex laser diode types. Most prevalent was the high current density that required the diode to operate at very low temperatures.
- Heterostructure Laser Diodes:
Hererostructure laser diodes are constructed of multiple n and p type materials. The double heterostructure (DH) diode laser is one of the most common types of laser diodes in use today. Unlike their homojunction counterpart they have a lower current density and are operable at room temperatures. This type of laser diode also sees an increase in efficiency (see image below).
- "- Photon confinement in the GaAs active region due to the larger index of refraction of GaAs (n = 3.6) compared to the p- and n- cladding layers (n = 3.4).
- -Carrier confinement in the GaAs active region due to the smaller band gap (Eg ≈ 1.5 eV)of the GaAs compared to the p- and n- cladding layers (Eg ≈ 1.8eV).
- Reduction in photon absorption arising from the differences in band gap of the active and cladding layers. Only photons created with energy equal to or greater than the larger band gap cladding layer are absorbed. This results in only minor absorption at the blue tail of the emission profile."[10]
- Quantum Well Laser Diodes (QWLD):
Quantum Well laser diodes are a class of DH diode in which the active middle region is reduced in thickness. These laser diodes feature a current density 4-5 times lower than DH diodes and possess a higher differential gain that is less susceptible to thermal variations[10]. QW laser diodes can also be found in what is known as a Multiple Quantum Well (MQW) and Strained Quantum Well (SQW) configurations. MQW diodes possess a series of alternating narrow and wide band gap materials. SQW diodes are QW diodes that have a lattice structure mismatch.
- Distributed Feedback Lasers (DFB):
These laser diodes feature an internal diffraction grating that is etched close to the active region of the diode. This grating acts as a filter and reflects a predetermined wavelength back into the gain region. This removes the requirement that the ends or facets of the diode be polished to act like mirrors. The main feature of this type of laser diode is its stable output frequency determined by the grating.
- Vertical Cavity Surface-Emitting Laser Diode (VCSELs):
VCSEL diodes possess a vertical optical cavity as opposed to the laterally oriented cavity of the previous laser diodes. In this orientation light travels parallel to the direction of current. The active layers are typically composed of multiple SQW layers between quarter-wave Bragg reflectors. Due to the short gain length the output power of these diodes is typically less than other laser diodes. However, this configuration allows for a greater density of emitters.
- External Cavity Diode Laser (ECDL):
In an ECDL the laser diode has one facet coated with an anti-reflection coating. The output from this facet is then directed through a collimating lens to an external mirror that completes the optical cavity of the laser. This type of laser is also often known as a tunable diode laser since the output wavelength can be adjusted via external mirrors and diffraction gratings. There are two configuration types that are common with ECDLs. These are the Littrow configuration and the Littman-Metcalf configuration. In the Littrow configuration the external cavity consists of the collimating lens and a diffraction grating with the laser output coming directly from the grating. Adjusting the wavelength is accomplished by rotating the diffraction grating. The disadvantage of this is that the output direction changes as the diffraction grating is adjusted. In the Littman-Metcalf configuration a mirror is added to the external cavity in addition to the collimating lens and the diffraction grating. Wavelength adjustment is carried out by adjusting the mirror and the output comes from the stationary diffraction grating. This alleviates the problem with output direction that characterized the Littrow configuration but comes at the cost of reduce power due to the loss of the zero-order reflection by the tuning mirror.
Figure 10: ECDL Configurations [6]
- Characteristics:
Laser diodes come in a wide variety of wavelength and power configurations. Combine this with their size and efficiency and offer a very wide range of application. They can, however, be difficult to control and the light they emit possesses characteristics that are often undesirable.
- Coherence:
Due to the small cavity length and facet geometry a typical laser diode emits an elliptical cone of light. This divergence is often measured with the full width-half maximum (FWHM) light power in the perpendicular and parallel axis with regard to the active region of the laser diode. Typical values are in the area of 30 in the parallel axis and 10 in the perpendicular axis [6]. To correct this, a collimating lens is typically employed to collimate the beam and reduce divergence. Laser diodes also suffer from a slight astigmatism as a result of the unequal divergence of the beam. The wide divergence places the source of the beam much closer to the facet than the narrower divergence.
- Power:
Laser diodes come in a wide array of power outputs depending on the application. Lower power applications such as CD players and laser pointers typically have a laser diode less than 1 mW to 5 mW. Laser printers may can use laser diodes anywhere from 5 mW to over 50 mW depending on the speed and resolution of the printer[13]. HD DVD and Blu-Ray burners can contain laser diodes from 100 mW to 500 mW. The laser diodes with power ratings greater than 500 mW are typically found in laboratory and industrial settings and can reach 100s of Watts.