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1 Introduction

1.1 Background of research anddevelopment at the CRL

The development of the hydrogen maser atthe Communications Research Laboratory(CRL) began in 1965. In the following year(1966), the CRL succeeded in producing thethird hydrogen maser oscillator in the world,after the United States and Switzerland. TheCRL subsequently made its research resultsavailable to Anritsu Corporation, permittingthe latter to place commercial hydrogenmasers on the market, which are now in use atvarious research facilities in Japan.

In addition, recently the CRL has beendeveloping a space-borne hydrogen maser fora global positioning system in collaborationwith Anritsu Corporation, publicizing itsresearch results at a variety of national andinternational academic gatherings.

1.2 Features of hydrogen maser andcomparison to other atomic frequencystandards

In addition to the hydrogen-maser frequen-cy standard, major atomic frequency standardsin practical use include the following:- Rubidium atomic frequency standard- Cesium atomic frequency standard

Fig.1 shows the general frequency sta-

bilites of atomic frequency standards.The rubidium atomic frequency standard

uses a gas cell in which rubidium atoms areconfined; it can be miniaturized and madelightweight, and is widely applicable as a ref-erence signal source in the communicationsand broadcasting fields. The rubidium atomicfrequency standard is inferior in terms of long-term stability relative to the cesium atomicfrequency standard, although its frequencystability in the short term is comparable withthat of the latter.

The cesium atomic frequency standardcreates a reference frequency by exciting heat-ed cesium atoms in a vacuum and causing theexcited atoms to interact with a microwave.This standard is characterized by its superiorlong-term stability, and has led to a scientificdefinition of the second based on the transitionfrequency of a cesium atom. As a result, stan-dards research organizations in Germany,France, Italy, the U.S., and Japan operatecesium atomic primary frequency standards,providing reports on accuracy to the BureauInternational des Poids et Mesures (BIPM),contributing to the overall accuracy of TAI.The CRL also operates an optical-pumpingcesium atomic primary frequency standard(CRL-O1), and reports on its accuracy to theBIPM several times a year.

On the other hand, atomic-fountain-type

ITO Hiroyuki et al.

3-4 Hydrogen Maser

ITO Hiroyuki, HOSOKAWA Mizuhiko, UMEZU Jun, MORIKAWA Takao,TSUDA Masahiro, TAKAHEI Ken-ichiro, UEHARA Masaro, and MORI Kenjiro

Communications Research Laboratory (CRL) plays an important role in development ofhydrogen maser in Japan. In this paper we describe the history of development of thehydrogen maser in CRL, and the principle of the hydrogen maser. Further, as a recent topic,we also describe the development of the space-borne hydrogen maser.

Keywords Atomic frequency standard, Hydrogen maser, Space-borne hydrogen maser, Frequen-cy stability, Sapphire loaded cavity

86

cesium primary frequency standards haverecently begun to be promoted as being capa-ble of greater frequency accuracy. With thesestandards a group of atoms is trapped by lasercooling and made to interact with amicrowave. This method is expected to leadto a nearly tenfold increase in accuracy rela-tive to the conventional thermal-beam method.

The most significant feature of the hydro-gen maser frequency standard is that its short-term stability is extremely high relative toother atomic frequency standards. This stan-dard is thus suited for use as a signal sourcefor very long baseline interferometers (VLBIs)and as a signal source for atomic primary fre-quency standards.

Generally the hydrogen maser is large andheavy, as it consists of a microwave cavity formaser oscillation, a vacuum pumping system,and a hydrogen source, in addition to othercomponents.

1.3 Development of space-bornehydrogen maser

The first global positioning system, orGPS, was originally developed for militarypurposes, but today has found extremely wideapplications: in automobile navigation, civilengineering, construction surveying, oil explo-ration, logistics of taxi and truck operations,air traffic control, prediction of earthquakes,GPS meteorology, and much more. Thus, theGPS today forms an essential part of the infra-structure indispensable to modern society,

with its fields of application rapidly expandingthroughout a wide range of markets. Never-theless, Japan's global positioning applicationsdepend entirely on the U.S. GPS; the nationhas as yet not developed any of its own tech-nologies.

This has been presented as a problem, inthat Japan depends entirely on another countryfor such an important system. Recentlydemands have increased for Japan to developits own global positioning technologies. Inthis context the Satellite Positioning Technolo-gy Working Group of the Space ActivitiesCommission submitted a report entitled"Guidelines for Satellite Positioning Technol-ogy Development in Japan" in 1997, recom-mending that our country establish the basictechnologies of a global positioning system inthe short term, to be tested with a recommend-ed minimum number of satellites.

The report enumerated the following threesuch basic technologies: (1) technology for aspace-borne atomic frequency standard, (2)time control technology for an ensemble ofsatellites, and (3) highly precise satellite orbit-determining technology. Based on this report,the Communications Research Laboratory(CRL) and the National Space DevelopmentAgency of Japan have begun related research.In particular, in connection with the space-borne atomic frequency standard, the CRL hasbeen developing space-borne hydrogen masers(SHM) in collaboration with Anritsu Corpora-tion, based on research and development[1][2]since fiscal 1997.

Space-borne specifications present numer-ous technological problems, such as harshtemperatures and severe mechanical vibrationat launch. As a result, hydrogen masers haveyet to be subject to operations in space (withthe exception of a rocket-based ballistic flight[5] performed in the 1970s to detect gravityshift). However, recently Europe and Americahave begun to develop SHMs for scientificapplications such as space VLBIs and verifi-cation of the general theory of relativity, aswell as applications in next-generation satel-lite positioning systems and space stations [6].

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003

Stability of atomic frequency standardFig.1

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2 Principle and construction

2.1 Principle and constructionThe basic construction of the hydrogen

maser is shown in Fig.2. First, hydrogen gasfrom a hydrogen source is controlled by adevice to maintain a constant flow rate; mole-cules of the gas are then dissociated intoatoms by high-frequency electric discharge.The hydrogen atoms are given directivity by acollimator, and are emitted into a vacuum asan atomic beam. Only the atoms in the upperlevel are required for maser oscillation; theseare selected via a level-selecting magnet andare ejected toward a microwave cavity.Hydrogen atoms remain in a storage spacewithin the cavity for approximately 1 sec.During this period, each atom performing atransition to a lower level emits electromag-netic wave energy of 1.42 GHz, generatingmaser oscillation. An external VCXO isphase-locked to this signal to create a standardfrequency. This summarizes the basic princi-ple of the hydrogen maser.

2.2 Frequency fluctuation factorsSince the oscillation frequency of the

hydrogen maser fm may fluctuate due to vari-ous physical factors, in order to attain suffi-

cient frequency stability, these fluctuation fac-tors must be taken into consideration indesign. The fluctuation factors of fm includethose that originate from noise (and are thusrandom) and those that are systematic. Thelatter factors will be considered here. fm isexpressed by the following formula.

(1)

In the formula, fH is the transition frequen-cy of the hydrogen atom, fc is the frequency ofthe microwave cavity, Qc is the loaded qualityfactor of the microwave cavity, and Ql is thespectrum line quality factor of the maser oscil-lation. Therefore, as fluctuation factors of fm,variations in fc, fH, Qc, Ql should be taken intoconsideration, while variations in Qc,Ql aresufficiently small and need not be consideredhere.

Since fluctuation of fc causes fluctuation offm, as seen in formula (1), high stability isrequired for fc. Usually, Qc and Ql are approxi-mately Qc～40,000, Ql～1×109, and it is nec-essary to satisfy dfc/fc < 2.5×10-11 (dfc <0.04Hz) in order to achieve a frequency stabil-ity of 1×10-15.

The most significant fluctuation factors offc consist of cavity deformation caused by achange in cavity temperature Tc and variationin the dielectric constant of the storage valve.dfc/dTc depends on a variety of variables,including the structure of the cavity, the coef-ficient of thermal expansion of materials, andthe temperature coefficient of the dielectricconstant of the valve material. dfc/dTc of a nor-mal full-size cavity having a TE011 mode is －1～－0.3kHz/K. In order to set dfc < 0.04Hz, it isnecessary to satisfy dTc < 4×10-5～1.3×10-4K,which requires severe temperature control.

Major shift factors of fH include the sec-ond-order Zeeman shift, the spin exchangeshift, the second-order Doppler shift, and thewall shift. If any one of these factors is sub-ject to variation, fm will vary. Thus, stabilizingthese shift factors is essential in attaining ahigh degree of frequency stability.

In order to resolve the degeneration of the

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Structural drawing of hydrogen maserFig.2

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energy levels of the hydrogen atom, a constantstatic magnetic field is applied. fH shiftsaccording to the static magnetic field, asexpressed by the following formula (second-order Zeeman shift).

(2)

Here, fH0 is the transition frequency whenBc = 0, and Bc is the flux density of the staticmagnetic field, expressed in units of T. Thechange in transition frequency dfH when Bc

changes by dBc is expressed by the followingformula.

(3)

The hydrogen maser is operated with Bc

set to 0.1μT. In order to control dfH / fH0 to be1×10-15, the following conditions must be sat-isfied, in accordance with formula (3).

There are two fluctuation factors of Bc:temperature variation of the current source forthe static magnetic field coil, and variation inthe external magnetic field. The former canbe made sufficiently small, as the temperaturestability of recent DA converters is approxi-mately ±10ppm or better.

Collision of hydrogen atoms in the storagevalve shifts fH, as expressed by the followingformula.

(6)

Here,λ' is the spin exchange frequencyshift cross-section (4.1×10-20m2), h is Planck'sconstant, νr is the mean relative velocity ofhydrogen atoms,μ0 is the permeability in vac-uum,μB is the Bohr magneton, T20 is the trans-verse relaxation time constant when thehydrogen atomic beam has a value of zero, qis the oscillation quality factor of the hydrogenmaser, I is the intensity of the hydrogen atom-ic beam, and Ith is the intensity of the thresholdbeam. ΔfH is on the order of 10-13. In order to

obtain a frequency stability of 10-15, it is neces-sary to maintain the variation in I at 1% orless.

The second-order Doppler shift and thewall shift depend on storage valve tempera-ture. All that is required to attain a frequencystability of 1×10-15 is to stabilize the storagevalve temperature at about 0.01 K for bothshifts. Since the cavity temperature is gener-ally controlled with a stability of 10-4˚C orless, neither of these effects poses a problem.

As described above, the frequency fluctua-tion factors for a hydrogen maser differ innature, and thus stabilization measures must betailored to each individual fluctuation factor.

2.3 Purpose and function of each partThe operation of an atomic frequency

standard principally entails the following threestages.(1) Preparation of particles(2) Confinement of particles(3) Observation of particles

First, particles are prepared to obtain a suf-ficient difference between the numbers ofatoms at different levels in order to observethe net effect of the transition. The hydrogenmaser utilizes the transition between magnet-ic-field-induced hyperfine levels of the groundstate of a hydrogen atom; it is therefore firstnecessary to dissociate the hydrogen mole-cules to create hydrogen atoms. In turn onlyatoms that have transitioned to the upperhyperfine level are to be supplied to themicrowave cavity. The foregoing is imple-mented using high-frequency discharge (todissociate the molecule into atoms), a collima-tor (to create the directional atomic beam),and a level-selecting magnet (to select onlyatoms in the upper level).

In the next stage the particles are confinedin the interaction region for a sufficiently longperiod, thereby causing the particles to transi-tion, resulting in a signal with a narrowlinewidth. The linewidth of the set of particlesthat interact with the radiation field is givenby the following approximation formula.

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003

(4)

(5)

89

(7)

Here, Tr is the average interaction time.As seen in the above equation, Tr must belengthened to the full extent possible in orderto obtain a signal with a narrow linewidth; thisis done in the hydrogen maser through theconfinement of particles using the storagevalve.

In the observation of the particles in thefinal stage, an actual signal is obtained. Notethat the hydrogen maser uses maser oscillationgenerated by induced radiation in themicrowave cavity as a signal source, in con-trast to the cesium frequency standard and therubidium frequency standard. Also note thatalthough some hydrogen masers operate pas-sively, the present paper discusses only hydro-gen masers of the active type.

3 Constructing a space-bornehydrogen maser

3.1 Required per formance of aspace-borne hydrogen maser

The required performance of a space-borne hydrogen maser depends greatly on theorbit of the satellite to carry it, the environ-ment in which it operates, and the specifica-tions of the positioning system in which it isinstalled. Since these specifications have yetto be defined as specific values, we haveassumed the provisional values shown belowin our research to establish the technologiesnecessary for the development of the space-borne maser. As the system takes shape, actu-al specifications will be determined based onthe results of this research.

3.2 Technological challenges in spacedeployment

The CRL has been conducting technologi-cal development of the space-borne hydrogenmaser (SHM) in collaboration with AnritsuCorporation since 1997. Specifically, to datethe CRL has built a breadboard model (BBM)of the space-borne hydrogen maser and hascarried out a variety of performance tests.

While the hydrogen maser featuresmarkedly superior frequency stability relativeto other atomic clocks (such as rubidium andcesium clocks), as described above, it is inferi-or in terms of weight, size, power consump-tion, and others. The technology of the hydro-gen maser itself has already matured, withAnritsu Corporation producing commercialmodels based on CRL research results[3][4],models used by major research facilitiesthroughout Japan. However, these productsare designed for use on the ground, weighingseveral hundred kilograms, and thus cannot becarried on satellites as they are. Therefore,when designing a space-borne hydrogenmaser, miniaturization and weight reductionare of primary importance. It is also necessaryto devise a structure capable of enduring themechanical vibrations of lift-off. Further-more, as electric power is strictly limited inspace, it is essential to design a device thatconsumes a minimum of power.

ITO Hiroyuki et al.

SHM specifications currently assumedTable 1

Breadboard model of space-bornemicro hydrogen maser

Fig.3

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3.3 Making the hydrogen maser smalland lightweight

The most significant factor determiningthe dimensions of the hydrogen maser is thesize of the microwave cavity. If the cavity canbe reduced in size, the external bell jar andmagnetic shield may in turn be reduced, andthe entire hydrogen maser can be made to besmall and lightweight. In the space-bornehydrogen maser, this reduction in size andweight has been achieved through the use of asapphire-loaded dielectric cavity.

In our previous analysis we found that thefrequency stability of the maser is best whenthe inside diameter of the cavity 2a is twice aslarge as the outside diameter of the sapphirecylinder 2b; we also found that the ratio of thecavity heights l and a can be determined interms of the reduction in weight of the maser,and that the optimal value of this ratio is 2. Atthe same time, the minimum cavity volume isdetermined based on the required frequencystability.

The sapphire-loaded dielectric cavityshown in Fig.4 was constructed in accordancewith these requirements to be capable of secur-ing frequency stability on the order of 10-15.While conventional cavity dimensions areapproximatelyφ300×300mm, the dimensionsof the sapphire-loaded cavity are φ161.9×161.9mm, resulting in miniaturization of about1/8 by volume.

Along with additional design optimization,the miniaturization of the cavity enabledminiaturization of the magnetic shield, the

weight of which was reduced to 17.4kg from64kg; i.e., to nearly 1/4 the original weight.

As further measures to promote miniatur-ization and reduced weight, an aluminum vac-uum chamber and a getter pump (for the vacu-um system) were employed, and a hydrogenstorage alloy was used as a hydrogen source.This allowed for significant reduction in theweight of the trial breadboard model, to 72kg.We are currently considering even furthermeasures, such as the use of so-called "honey-comb" materials.

3.4 Improved protection from thespace environment

A hydrogen maser to be deployed in spacewill operate under conditions more severethan those found on Earth. A number of pro-tective measures should be taken into consid-eration, including improved temperatureresistance, improved magnetic-field character-istics, improved resistance to mechanicalvibration during launch, and improved opera-tional characteristics in a vacuum.

In terms of temperature resistance, in thesapphire-loaded dielectric cavity, temperaturevariation in the dielectric constant of sapphireis large, dfc/dTc reaching -70.9 kHz/K. It istherefore almost impossible to stabilize fc withtemperature isolation alone; instead, stabiliza-tion of fc by cavity auto tuning becomes indis-pensable.

Possible factors responsible for externalmagnetic-field variation include variation inthe geomagnetic field and magnetic-field leak-age from the magnetic gyroscope controllingsatellite attitude. The variation in the geomag-netic field cannot be estimated accurately ifthe satellite orbit is not defined, but for pur-poses of illustration it can be estimated as fol-lows: approximately ± 8.0×10-7 T in the caseof an orbital altitude of 20,000 km; and about± 1.8×10-5 T in the case of an orbital altitudeof 3,100 km. At this time, in order to maintaindBc at 2.6×10-11 T or lower, it is necessary toachieve a magnetic shield factor of 62,000 ormore and 1,400,000 or more, respectively.

In short, a space-borne hydrogen maser

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003

Structure of sapphire-loaded cavityFig.4

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faces environmental conditions considerablymore severe than those found on the ground,and accordingly stabilization requirements aremore acute.

3.5 Evaluation of characteristics3.5.1 Frequency stability

The BBM frequency stability measure-ment results are shown in Fig.5. The obtainedstability is worse than the targeted stability bya factor of about 2 or 3, which is attributed toinfluence of the frequency fluctuation causedby the temperature change described later. Inthis context we believe that improving the sta-bility of the cavity temperature will result inimproved frequency stability.

3.5.2 Temperature characteristics

The temperature characteristics were esti-mated by installing the BBM in a thermostaticoven, increasing the temperature of the ovenfrom 22˚C to 23˚C, and measuring the fre-quency fluctuation and other characteristics asthe temperature increased. As described in theprevious section, in the sapphire-loaded cavi-ty, stabilizing the hydrogen maser by automat-ic control of fc is essential. This stabilizationof fc is performed by automatic, carrier-freetuning. Table 2 shows the measurement

results.The obtained temperature dependency of

the frequency fluctuation is 3×10-14, which isabout ten times larger than the targeted value3×10-15. As seen in Table 2, the dependencyof the cavity temperature on environmentaltemperature is 0.02˚C/˚C, ten times larger thanthe targeted value, which is considered to beone of the major causes of the frequency fluc-tuation. The following may be proposed aspossible causes of the significant dependencyof cavity temperature on environmental tem-perature.(1) Insufficient gain in the temperature controlcircuit(2) Heat-insulation performance of SHM(3) Variation in thermo-electromotive force atconnector component

We are presently investigating the actualcauses so that the required improvements canbe made.

3.5.3 Magnetic-field characteristics

Measurement of magnetic-field variationcharacteristics was performed by installing theSHM in a Helmholtz coil. The Helmholtz coilwas configured such that magnetic fields canbe applied independently to the x, y, z axes.The measuring time was 10 minutes, theapplied magnetic fields were two differenttypes ± 1G, and the measurement frequencywas 1.4GHz. Automatic tuning of SHM wasdisabled. Table 3 shows the measurementresults.

Calculation of the magnetic shield factorof SHM results in a value of 200,000. Asshown in 2.2, this is sufficient to operate thehydrogen maser regularly at an orbital altitudeof 20,000km. In other words, there will be noproblem when the maser is operated at normal

ITO Hiroyuki et al.

Results of stability measurementFig.5

Measurement results of tempera-ture characteristics

Table 2

Frequency fluctuation upon appli-cation of magnetic field

Table 3

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orbital altitudes of more than 12,000 km.However, it may become necessary to increasethe magnetic shield factor further, dependingon the actual operational orbit.

3.5.4 Vibration testA vibration test was conducted on an

experimental prototype constructed to bephysically equivalent to the BBM. The exper-imental prototype was designed such that thevibration test could be conducted under thesame conditions as those that would applyusing the BBM, including factors such asweight distribution. Fig.6 shows the experi-mental configuration.

Measurement was performed with anacceleration of vibration of 5.0 G in a frequen-cy range of 5 to 300Hz. The frequency was

swept upward from 5Hz to 300Hz in two min-utes and subsequently downward from 300Hzto 5Hz in two minutes. Fig.7 shows the meas-ured results and Table 4 shows the major reso-nant frequencies obtained, in addition to theacceleration values applied to the prototype ateach resonant frequency.

Inspection of vacuum leakage and inspec-tion of the hydrogen beam axis were conduct-ed concurrently with the vibration test. Nosignificant vacuum leak was observed and thedeviation in the hydrogen beam axis wasfound to be 0.1mm or less.

In addition, a deviation of about 100kHzwas observed in the resonant frequencies ofthe microwave cavity after the vibration test,which was attributed to temperature variationof the cavity.

3.6 Future challengesPresently, the service life of the vacuum

pumping system imposes a limit on the servicelife of the space-borne hydrogen maser.Therefore, longer operation will be possible ifthe hydrogen atoms can be more efficientlysupplied to the storage valve. To this end weare now in the process of switching from asingle collimator to a multi-collimator, modi-fying the design of the level-selecting magnetat the same time.

In terms of temperature variation, initialdata indicated sufficient performance. How-ever, we must conduct more detailed experi-ments with a view to improved operations inpractical deployment.

As for the vibration test, several parts needto be redesigned to ensure that the maser iscapable of enduring a vibration test at 20G,followed by an operational test using an

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003

Configuration of vibration testFig.6

Results of vibration testFig.7

Experiment results of mechanicalvibration endurance test

Table 4

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experimental prototype capable of actualoscillation.

4 Concluding remarks

This paper outlines the hydrogen maseratomic frequency standard, describing itsbasic principles and construction, sets forththe research history at the CRL, and illustratesthe latest achievements in the development ofthe space-borne hydrogen maser.

Due to its high frequency stability, thehydrogen maser is expected to gain evenwider use than seen today, most notablyincluding expansion to new space-based appli-cations.

The CRL has played a key role to date inthe development of the hydrogen maser inJapan, and, with its current focus on deploy-ment in space, will continue to assume a lead-ing role in research and development well intothe future.

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UMEZU Jun

Senior Researcher, Basic and AdvancedResearch Division

Microwave Engineering

HOSOKAWA Mizuhiko, Ph. D.

Leader, Atomic Frequency StandardsGroup, Applied Research and Stan-dards Division

Atomic Frequency Standards, SpaceTime Measurements

MORIKAWA Takao

Research Supervisor, Applied Researchand Standards Division

ITO Hiroyuki, Ph. D.

Researcher, Atomic Frequency Stan-dards Group, Applied Research andStandards Division

Atomic Frequency Standards

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 200394

TSUDA Masahiro

Senior Manager, Research Laborato-ries, Anritsu Corporation

Atomic Frequency Standards

UEHARA Masaro

Atomic Frequency Standard ProjectTeam, Research Laboratories, AnritsuCorporation

Atomic Frequency Standards

TAKAHEI Ken-ichiro, Ph. D.

Senior Research Engineer, ResearchLaboratories, Anritsu Corporation

Atomic Frequency Standards

MORI Kenjiro

Former Research Laboratories, AnritsuCorporation

Atomic Frequency Standards