JP4236990B2

JP4236990B2 - Method for producing porous preform for optical fiber

Info

Publication number: JP4236990B2
Application number: JP2003163388A
Authority: JP
Inventors: 久美永田; 成敏山田
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 2003-06-09
Filing date: 2003-06-09
Publication date: 2009-03-11
Anticipated expiration: 2023-06-09
Also published as: JP2004359531A

Description

【０００１】
【産業上の利用分野】
本発明は、光ファイバ用多孔質母材の成長に合わせて堆積される多孔質ガラス層の密度を、２段階の低減モードにより制御することで、効率よく母材が成長できるようにした光ファイバ用多孔質母材の製造方法に関するものである。
【０００２】
【従来の技術】
一般に、光ファイバ用多孔質母材の製造方法としては、ＶＡＤ法、外付け法などが知られていて、これらの製造方法に関する基本的な技術は、成熟し、既にほぼ確立されているものと言える。特に最近では、光通信の需要の増大に伴い、光ファイバの需要も年々増加する傾向にあるため、専ら製造コストの低減が大きな課題となってきている。
【０００３】
この製造コスト低減の観点から、光ファイバ用母材の大径化や長尺化が要求されてきている。母材の大径化にあたっては、大径の分だけ多孔質ガラス層が厚くなるため、製造段階で多孔質ガラス層が割れ易くなったり、透明ガラス化後、気泡が残ったり、或いは層剥離が起こるなどの問題があった。また、長尺化にあっては、ガラス微粒子合成用バーナの１回の移動ストロークが長くなるため、その間の制御が難しくなるなどの問題があった。
【０００４】
そこで、母材製造の大径化にあって、既に多孔質ガラス層の割れ（スート割れ）や、透明ガラス化後の気泡残留、層剥離などを抑制するため、デポジションの最初から最後まで、多孔質ガラス層の密度をほぼ一定に保持するのではなく、当初のターゲット部材の周辺では、多孔質ガラス層の密度を高めにして、その後、中心部から周辺部にかけて、なだらかに多孔質ガラス層の密度を低減させる方法が提案されている（特許文献１参照）。
【０００５】
【特許文献１】
特開平２−２０４３４０号（特許第２７９３６１７号）
【０００６】
ところが、上記多孔質ガラス層の密度を中心部から周辺部にかけてなだらかに低減させる方法においても、その効果（スート割れや、気泡残留、層剥離などの抑制効果）が不十分であるため、多孔質ガラス層の密度制御を、多孔質母材の堆積面の温度を制御することにより行い、さらに、この堆積面の温度制御は、燃料ガス流量の調整により行うことで、良好な効果が得られるとする方法も提案されている（特許文献２参照）。
【０００７】
【特許文献２】
特開２０００−２７２９２９号
【０００８】
【発明が解決しようとする課題】
しかしながら、上記いずれの方法の場合も、基本的には、デポジションの最初から最後まで、多孔質ガラス層の密度を一つのパターンで、次第に低減させて行く方法であるため、本発明者等の試験研究によると、以下のような問題点があることが分かった。
【０００９】
デポジションの初期にあっては、確かに、ターゲットの周辺で多孔質ガラス層の密度を高めにし、その後、中心部から周辺部にかけてなだらかに密度を低減させる方法が好ましいものの、母材が成長して所定の外径に達すると、それまで主に慣性力によって堆積されていたガラス微粒子に対して、当該ガラス微粒子を母材側に引き込む作用となる、サーモフォレシス効果も大きく働くようになるため、母材外径はより急速に成長するようになる。このサーモフォレシス効果というのは、ターゲット外径が大きくなることにより、ガラス微粒子合成用バーナから吹き出された火炎、即ちガラス微粒子がターゲットと接する時間（範囲）が大きくなるため、ガラス微粒子がターゲット側に引き寄せられるようなる作用と考えられている。
【００１０】
一方、特に母材の大径化にあっては、原料ガス流出口の大きさなガラス微粒子合成用バーナを用いて、より多くの原料ガスを供給することが行われるため、母材外径の急成長は、益々増長されることになる。この結果、母材外径が所定の大きさに達すると、多孔質ガラス層の密度が急速に小さくなり、デポジションの後期に至ると、母材外径が大きくなり過ぎて、既存の製造装置では、取り扱い難くなるという問題が生じるようになる。もちろん、この密度の急速な低下は、上述したスート割れや、気泡残留、層剥離などの発生要因ともなる。
【００１１】
このため、本発明者等は、母材が所定の外径に達するまでは、多孔質ガラス層の密度を一定の割合で低減させる一方、所定の外径に達した後は、多孔質ガラス層の密度を上記一定の低減割合よりさらに緩く低減させることの方が、製造工程全体から見れば、かえって有利になるのではないかと、着想するに至った。
【００１２】
この着想に基づいて、後述するように、種々の試験研究を行ったところ、上記のように、光ファイバ用多孔質母材の外径に応じた多孔質ガラス層の密度変化を直線近似させた場合において、デポジションの初期から母材が所定の外径に達するまでは、多孔質ガラス層の密度を直線的に低減させる第１の低減モードと、母材が所定の外径に達した後は、上記第１の低減モードよりさらに緩く直線的に低減させる第２の低減モードとすることで、良好な結果が得られることを見い出した。
【００１３】
さらに、母材の成長は、ガラス微粒子合成用バーナからの原料ガスの供給量、即ち、バーナの原料ガス流出口の大きさに左右されるため、母材の外径（Ｄ_g ）と原料ガス流出口の大きさ（Ｄ_s ）の比（Ｄ_g ／Ｄ_s ）から、多孔質ガラス層における密度の低減割合の変換点、即ち、第１の低減モードと第２の低減モードとの変換点が求められることを見い出した。そして、さらに、この比（Ｄ_g ／Ｄ_s ）が、１４≦Ｄ_g ／Ｄ_s ≦３６となるように調整すれば、良好な結果が得られることも見い出した。
【００１４】
本発明は、このような観点に立ってなされたものであり、基本的には、ガラス微粒子の堆積過程において、上記した第１の低減モードから第２の低減モードに切り換えることにより、結果として、より良好な母材の成長が得られるようにした光ファイバ用多孔質母材の製造方法を提供せんとするものである。
【００１５】
【課題を解決するための手段】
請求項１記載の本発明は、ガラス微粒子合成用バーナからのガラス微粒子をターゲット部材外周に堆積させて光ファイバ用多孔質母材を形成する光ファイバ用多孔質母材の製造方法であって、
前記光ファイバ用多孔質母材の外径に応じた多孔質ガラス層の密度変化を直線近似させた場合において、前記ガラス微粒子の堆積による光ファイバ用多孔質母材の成長する外径と重量をモニターして、多孔質ガラス層の密度を演算し、前記光ファイバ用多孔質母材が所定の外径に達するまでは、多孔質ガラス層の密度を一定の割合で直線的に低減させる第１の低減モードで前記ガラス微粒子を堆積させる一方、所定の外径に達した後は、多孔質ガラス層の密度を前記一定の低減割合よりさらに緩く直線的に低減させる第２の低減モードで前記ガラス微粒子を堆積させると共に、前記第１の低減モードから前記第２の低減モードへの変換は両低減モードのなす近似の両直線の交差する点とし、かつ、前記光ファイバ用多孔質母材の外径（Ｄ _g ）とガラス微粒子合成用バーナの原料ガス流出口の大きさ（Ｄ _s ）との比（Ｄ _g ／Ｄ _s ）が、１４≦Ｄ _g ／Ｄ _s ≦３６となるように調整することを特徴とする光ファイバ用多孔質母材の製造方法にある。
【００１６】
【発明の実施の形態】
図１は、本発明に係る光ファイバ用多孔質母材の製造方法を実施するための製造装置系の一例を示し、図２は、この製造装置系で用いられるガラス微粒子合成用バーナの一例を示したものである。
【００１７】
本発明では、ターゲット部材１０の両端をチャックなどの把持部２０，２０で保持しつつ、回転させる一方、例えば、２個のガラス微粒子合成用バーナ１００を、上記ターゲット部材１０に対峙させて、両者を相対的に移動させることにより、ターゲット部材外周にガラス微粒子合成用バーナ１００からのガラス微粒子（スート）１１を堆積させて、成長させる。
【００１８】
上記ガラス微粒子合成用バーナ１００の構造は、特に限定されないが、図２の場合、ＳｉＣｌ₄ などの原料ガス（通常酸素ガスなどのキャリアガスが添加されることが多い）が供給される原料ガス流出口１１０、窒素ガスなどの不活性ガスが供給される不活性ガス流出口１２０、水素ガスなどの可燃性ガスが供給される可燃性ガス流出口１３０、この可燃性ガス流出口１３０内に配置されて、酸素ガスなどの支燃性ガスが供給される複数の支燃性ガス流出口１４０、窒素ガスなどの不活性ガスが供給される最外周側の不活性ガス流出口１５０からなる。
【００１９】
このような構造のガラス微粒子合成用バーナ１００からの火炎１００ａをターゲット部材１０側に吹き付けて、その外周にガラス微粒子１１を堆積させるわけであるが、この際、本発明では、図３に示すように、光ファイバ用多孔質母材の外径に応じた多孔質ガラス層の密度変化を、直線的に近似させた２段階のモードで行うようにしてある。より具体的には、デポジションの初期から母材が所定の外径に達するまでは、ガラス微粒子１１の堆積層である、多孔質ガラス層の密度を、一定の割合で直線的に低減させる。つまり、ほぼ第１の低減モードに相当する仮想直線Ｉに沿って、多孔質ガラス層の密度を低減させる。そして、母材が所定の外径に達した後は、多孔質ガラス層の密度を、上記一定の低減割合よりさらに緩く直線的に低減させる。つまり、ほぼ第２の低減モードに相当する仮想直線ＩＩに沿って、多孔質ガラス層の密度を低減させる。なお、ここで、密度の仮想直線Ｉ〜ＩＩは、あくまでも理想的な密度制御の場合であって、実際の制御では、これらの仮想直線Ｉ〜ＩＩに添った密度の近似値であってもよい。
【００２０】
なお、本発明では、多孔質ガラス層の密度は、母材の成長、即ちその外径を光学的手段によりモニターすると共に、母材の重量もモニターし、コンピュータ内蔵の制御装置により適宜演算して求めている。
【００２１】
図３の場合、デポジションの初期に多孔質ガラス層の密度を、０．７５程度とし、母材の外径が７０ｍｍ程度になるまでは、０．２５程度まで直線的に低減させた後、２３０ｍｍ程度の最終的な外径になるまでは、０．２５〜０．２０程度の密度の間で、緩く直線的に低減させているが、本発明は、特にこれに限定されるものではない。つまり、母材外径の最終的な大きさや、ガラス微粒子合成用バーナ１００における原料ガス流出口１１０の大きさ、原料ガス流出速度などの種々のパラメータの相違により、ある程度の幅をもって制御することができる。
【００２２】
上記のように多孔質ガラス層の密度を第１の低減モードから第２の低減モードに変換する変換点（変曲点Ｃ）は、図３の場合、母材外径が７０ｍｍ程度のところとしてあるが、これも、上記したような製造上の種々のパラメータの相違により調整すべきものと考えられる。この点について、本発明等は、ガラス微粒子合成用バーナ１００の原料ガス流出口１１０の大きさ（Ｄ_s ）、即ち、この大きさ（Ｄ_s ）に起因する原料ガスの広がりと、堆積成長する母材の外径（Ｄ_g ）について、着目し、種々の試験研究を行ったことろ、後述する実施例から明らかなように、その比（Ｄ_g ／Ｄ_s ）から求められることが分かった。
【００２３】
そして、さらに、この比（Ｄ_g ／Ｄ_s ）が、１４≦Ｄ_g ／Ｄ_s ≦３６となるように調整すれば、良好な結果が得られることも分かった。つまり、Ｄ_g ／Ｄ_s が１４未満では、原料ガスの広がりに対して、母材の外径（Ｄ_g ）が小さ過ぎるため、上述したサーモフォレシス効果があまり期待できず、母材の成長が遅く、結果的にガラス微粒子の堆積効率が低下するようになるからである。一方、Ｄ_g ／Ｄ_s が３６を越える場合には、母材の中心部と外周部における多孔質ガラス層の密度差が大きくなり過ぎるため、この結果として、上述したように、スート割れや、透明ガラス化後の気泡残留、層剥離などの問題が生じるようになるからである。もちろん、多孔質ガラス層の密度が低くなると、母材外径が急速に大きくなるため、既存の製造装置系では取り扱い難くなるという問題も生じる。
【００２４】
このようなことから、本発明では、多孔質ガラス層の密度の上記変換点までは、上記第１の低減モードに沿う形で、ガラス微粒子合成用バーナ１００における種々のガス流量を調整して、なるべく速く母材の外径を所定の外径まで成長させる。そして、この後は、上記第２の低減モードに沿う形で、ガラス微粒子合成用バーナ１００における種々のガス流量を調整して、母材の最終的な外径まで成長させる。これにより、製造工程全体から見れば、後述するように、ガラス微粒子の最終的な平均堆積効率及び平均堆積速度が向上するため、結果として、優れた生産性が得られる。
【００２５】
〈実施例１〉
上記図１の製造装置系、及び図２と同構造のガラス微粒子合成用バーナを用いて、外径（直径）が３０ｍｍφのターゲット部材の外周にＳｉＯ₂ のガラス微粒子を１５Ｋｇ堆積させて、光ファイバ用多孔質母材を得た。
【００２６】
このとき、ガラス微粒子合成用バーナにおける原料ガス流出口の大きさ（Ｄ_s ）は５．０ｍｍで、原料ガスはＳｉＣｌ₄ 、キャリアガスは酸素ガス、可燃性ガスは水素ガス、支燃性ガスは酸素ガス、不活性ガスは窒素ガスをそれぞれ用いた。そして、原料ガス流出口からは、原料ガスとキャリアガスの混合ガス流量を、ガス流速が９．８ｍ／ｓｅｃとなるように調整した。一方、燃料ガスとしての流量は、即ち、可燃性ガスの流量を流速が１．３〜１．７ｍ／ｓｅｃ、支燃性ガスの流量を流速が１０．１〜１３．０ｍ／ｓｅｃとなるように調整した。また、不活性ガスの流量は流速が０．７ｍ／ｓｅｃとなるように調整した。
【００２７】
この調整中、母材の外径と母材の重量のモニター情報により、多孔質ガラス層の密度と、上記比（Ｄ_g ／Ｄ_s ）を常時演算し、デポジションの初期から母材外径が所定の外径になるまでは、上記した如き、第１の低減モードに相当する仮想直線Ｉに沿って、多孔質ガラス層の密度を低減させる一方、所定の外径に達した後は、上記した如き、第２の低減モードに相当する仮想直線ＩＩに沿って、多孔質ガラス層の密度を低減させた。このとき、多孔質ガラス層の密度の変換点は、Ｄ_g ／Ｄ_s ＝２０．２のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２７．３％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６８％、平均堆積速度は２８．０ｇ／ｍｉｎであった。
【００２８】
〈実施例２〉
上記実施例１とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝１４．３のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２５．５％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６５％、平均堆積速度は２６．３ｇ／ｍｉｎであった。
【００２９】
〈実施例３〉
上記実施例１とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝３５．３のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２４．２％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６０．５％、平均堆積速度は２３．８ｇ／ｍｉｎであった。
【００３０】
〈比較例１〉
上記図１の製造装置系、及び図２と同構造のガラス微粒子合成用バーナを用いて、外径（直径）が３０ｍｍφのターゲット部材の外周にＳｉＯ₂ のガラス微粒子を１５Ｋｇ堆積させて、光ファイバ用多孔質母材を得た。
【００３１】
このとき、ガラス微粒子合成用バーナにおける原料ガス流出口の大きさ（Ｄ_s ）は５．０ｍｍで、原料ガスはＳｉＣｌ₄ 、キャリアガスは酸素ガス、可燃性ガスは水素ガス、支燃性ガスは酸素ガス、不活性ガスは窒素ガスをそれぞれ用いた。そして、原料ガス流出口からは、原料ガスとキャリアガスの混合ガス流量を、ガス流速が９．８ｍ／ｓｅｃとなるように調整した。一方、燃料ガスとしての流量は、即ち、可燃性ガスの流量を流速が１．５〜２．１ｍ／ｓｅｃ、支燃性ガスの流量を流速が１１．７〜１４．８ｍ／ｓｅｃとなるように調整した。また、不活性ガスの流量は流速が０．７ｍ／ｓｅｃとなるように調整した。
【００３２】
この調整中、母材の外径と母材の重量のモニター情報により、多孔質ガラス層の密度と、上記比（Ｄ_g ／Ｄ_s ）を常時演算し、デポジションの初期から母材外径が所定の外径になるまでは、上記した如き、第１の低減モードに相当する仮想直線Ｉに沿って、多孔質ガラス層の密度を低減させる一方、所定の外径に達した後は、上記した如き、第２の低減モードに相当する仮想直線ＩＩに沿って、多孔質ガラス層の密度を低減させた。このとき、多孔質ガラス層の密度の変換点は、Ｄ_g ／Ｄ_s ＝１２．８のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２２．８％を示し、母材の成長が遅く、最終的な平均堆積効率は５０％、平均堆積速度は２２．０ｇ／ｍｉｎであった。
【００３３】
〈比較例２〉
上記比較例１とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝３９．８のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２３．８％を示したが、母材の中心部と外周部との密度差が大きくなり過ぎて、スート割れや、透明ガラス化後の気泡残留、層剥離などの発生が見られた。
【００３４】
〈実施例４〉
上記図１の製造装置系、及び図２と同構造のガラス微粒子合成用バーナを用いて、外径（直径）が３０ｍｍφのターゲット部材の外周にＳｉＯ₂ のガラス微粒子を１５Ｋｇ堆積させて、光ファイバ用多孔質母材を得た。
【００３５】
このとき、ガラス微粒子合成用バーナにおける原料ガス流出口の大きさ（Ｄ_s ）は３．０ｍｍで、原料ガスはＳｉＣｌ₄ 、キャリアガスは酸素ガス、可燃性ガスは水素ガス、支燃性ガスは酸素ガス、不活性ガスは窒素ガスをそれぞれ用いた。そして、原料ガス流出口からは、原料ガスとキャリアガスの混合ガス流量を、ガス流速が２４．３ｍ／ｓｅｃとなるように調整した。一方、燃料ガスとしての流量は、即ち、可燃性ガスの流量を流速が１．４〜２．１ｍ／ｓｅｃ、支燃性ガスの流量を流速が１３．４〜１７．５ｍ／ｓｅｃとなるように調整した。また、不活性ガスの流量は流速が０．７ｍ／ｓｅｃとなるように調整した。
【００３６】
この調整中、母材の外径と母材の重量のモニター情報により、多孔質ガラス層の密度と、上記比（Ｄ_g ／Ｄ_s ）を常時演算し、デポジションの初期から母材外径が所定の外径になるまでは、上記した如き、第１の低減モードに相当する仮想直線Ｉに沿って、多孔質ガラス層の密度を低減させる一方、所定の外径に達した後は、上記した如き、第２の低減モードに相当する仮想直線ＩＩに沿って、多孔質ガラス層の密度を低減させた。このとき、多孔質ガラス層の密度の変換点は、Ｄ_g ／Ｄ_s ＝２１．３のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２８．４％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６１．８％、平均堆積速度は２５．５ｇ／ｍｉｎであった。
【００３７】
〈実施例５〉
上記実施例４とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝１４．８のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２８．７％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６３％、平均堆積速度は２５．２ｇ／ｍｉｎであった。
【００３８】
〈実施例６〉
上記実施例４とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝３５．７のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２７．８％を示し、その後も順調な増加を示し、最終的な平均堆積効率は６０％、平均堆積速度は２３．８ｇ／ｍｉｎであった。
【００３９】
〈比較例３〉
上記図１の製造装置系、及び図２と同構造のガラス微粒子合成用バーナを用いて、外径（直径）が３０ｍｍφのターゲット部材の外周にＳｉＯ₂ のガラス微粒子を１５Ｋｇ堆積させて、光ファイバ用多孔質母材を得た。
【００４０】
このとき、ガラス微粒子合成用バーナにおける原料ガス流出口の大きさ（Ｄ_s ）は３．０ｍｍで、原料ガスはＳｉＣｌ₄ 、キャリアガスは酸素ガス、可燃性ガスは水素ガス、支燃性ガスは酸素ガス、不活性ガスは窒素ガスをそれぞれ用いた。そして、原料ガス流出口からは、原料ガスとキャリアガスの混合ガス流量を、ガス流速が２４．３ｍ／ｓｅｃとなるように調整した。一方、燃料ガスとしての流量は、即ち、可燃性ガスの流量を流速が１．７２ｍ／ｓｅｃ、支燃性ガスの流量を流速が１６．２ｍ／ｓｅｃとなるように調整した。また、不活性ガスの流量は流速が０．７ｍ／ｓｅｃとなるように調整した。
【００４１】
この調整中、母材の外径と母材の重量のモニター情報により、多孔質ガラス層の密度と、上記比（Ｄ_g ／Ｄ_s ）を常時演算し、デポジションの初期から母材外径が所定の外径になるまでは、上記した如き、第１の低減モードに相当する仮想直線Ｉに沿って、多孔質ガラス層の密度を低減させる一方、所定の外径に達した後は、上記した如き、第２の低減モードに相当する仮想直線ＩＩに沿って、多孔質ガラス層の密度を低減させた。このとき、多孔質ガラス層の密度の変換点は、Ｄ_g ／Ｄ_s ＝１２．０のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２４．５％を示し、母材の成長が遅く、最終的な平均堆積効率は４８％、平均堆積速度は２０．２ｇ／ｍｉｎであった。
【００４２】
〈比較例４〉
上記比較例３とほど同一の条件下で、デポジションを行い、このときの多孔質ガラス層の密度の変換点は、上記と同様のモニター情報から演算により、Ｄ_g ／Ｄ_s ＝３９．２のところで行った。
この結果、ガラス微粒子の堆積開始直後の堆積効率は２３．８％を示したが、母材の中心部と外周部との密度差が大きくなり過ぎて、スート割れや、透明ガラス化後の気泡残留、層剥離などの発生が見られた。
【００４３】
【発明の効果】
以上の説明から明らかなように、本発明に係る光ファイバ用多孔質母材の製造方法によると、光ファイバ用多孔質母材の外径に応じた多孔質ガラス層の密度変化を直線近似させた場合において、光ファイバ用多孔質母材が所定の外径に達するまでは、多孔質ガラス層の密度を一定の割合で直線的に低減させる、第１の低減モードから、所定の外径に達した後は、多孔質ガラス層の密度を前記一定の低減割合よりさらに緩く直線的に低減させる、第２の低減モードに切り換えることにより、結果として、製造工程全体から見れば、ガラス微粒子の最終的な平均堆積効率、及び平均堆積速度が向上するため、優れた生産性が得られる。
【図面の簡単な説明】
【図１】本発明に係る光ファイバ用多孔質母材の製造方法を実施するための製造装置系の一例を示した概略説明図である。
【図２】図１の製造装置系に用いられるガラス微粒子合成用バーナの一例を示した端面図である。
【図３】本発明に係る光ファイバ用多孔質母材の製造方法における、多孔質ガラス層の密度の制御例を示した概略説明図である。
【符号の説明】
１０ターゲット部材
１１ガラス微粒子
２０把持部
１００ガラス微粒子合成用バーナ
１００ａ火炎
１１０原料ガス流出口[0001]
[Industrial application fields]
The present invention provides an optical fiber capable of efficiently growing a base material by controlling the density of a porous glass layer deposited in accordance with the growth of the porous base material for an optical fiber by a two-stage reduction mode. It is related with the manufacturing method of the porous preform | base_material.
[0002]
[Prior art]
In general, the VAD method, the external method, and the like are known as methods for producing a porous preform for optical fibers, and the basic techniques relating to these production methods are mature and almost already established. I can say that. In particular, recently, with the increase in demand for optical communication, the demand for optical fibers tends to increase year by year, and thus reduction of manufacturing costs has become a major issue.
[0003]
From the viewpoint of reducing the manufacturing cost, it has been required to increase the diameter and length of the optical fiber preform. In increasing the diameter of the base material, the porous glass layer becomes thicker by the size of the larger diameter, so that the porous glass layer is easily broken at the manufacturing stage, or bubbles are left after separation into a transparent glass, or delamination occurs. There were problems such as happening. Further, in the case of lengthening, there has been a problem that the one-time movement stroke of the burner for synthesizing the fine glass particles becomes long, so that the control during that time becomes difficult.
[0004]
Therefore, in order to suppress the cracking of the porous glass layer (soot cracking), the bubble remaining after transparent vitrification, delamination, etc. Rather than keeping the density of the porous glass layer almost constant, the density of the porous glass layer is increased around the original target member, and then gently from the center to the periphery. There has been proposed a method for reducing the density (see Patent Document 1).
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 2-204340 (Japanese Patent No. 2793617)
[0006]
However, even in the method of gently reducing the density of the porous glass layer from the central part to the peripheral part, the effect (suppressing effect of soot cracking, bubble residue, delamination, etc.) is insufficient. Density control of the glass layer is performed by controlling the temperature of the deposition surface of the porous base material, and furthermore, by controlling the temperature of the deposition surface by adjusting the fuel gas flow rate, a good effect can be obtained. There has also been proposed a method (see Patent Document 2).
[0007]
[Patent Document 2]
JP 2000-272929 A
[Problems to be solved by the invention]
However, in any of the above methods, basically, since the density of the porous glass layer is gradually reduced in one pattern from the beginning to the end of the deposition, the present inventors According to the test study, the following problems were found.
[0009]
In the initial stage of deposition, it is certainly preferable to increase the density of the porous glass layer around the target and then gradually reduce the density from the center to the periphery, but the base material grows. When the predetermined outer diameter is reached, the thermophoresis effect, which is the action of drawing the glass fine particles to the base material side with respect to the glass fine particles that have been deposited mainly by inertia until then, will also work greatly. The base metal outer diameter grows more rapidly. This thermophoresis effect is because the flame blown out from the glass fine particle synthesis burner, that is, the time (range) for the glass fine particles to contact the target is increased by increasing the outer diameter of the target. It is considered to be an action that is attracted.
[0010]
On the other hand, particularly in increasing the diameter of the base material, a larger amount of source gas is supplied using a glass fine particle synthesis burner having a larger size of the source gas outlet, so that the outer diameter of the base material is increased. Rapid growth will increase further. As a result, when the outer diameter of the base material reaches a predetermined size, the density of the porous glass layer rapidly decreases. When the latter stage of deposition is reached, the outer diameter of the base material becomes too large, and the existing manufacturing apparatus Then, the problem that handling becomes difficult arises. Of course, this rapid decrease in density also causes the above-mentioned soot cracking, bubble residue, delamination, and the like.
[0011]
For this reason, the inventors reduce the density of the porous glass layer at a constant rate until the base material reaches a predetermined outer diameter, and after reaching the predetermined outer diameter, the porous glass layer The inventors have come up with the idea that it would be more advantageous to reduce the density of the material more slowly than the above-mentioned fixed reduction rate from the viewpoint of the entire manufacturing process.
[0012]
Based on this idea, various test studies were conducted as described later. As described above, the density change of the porous glass layer according to the outer diameter of the optical fiber porous preform was linearly approximated. In the case, from the initial stage of deposition until the base material reaches a predetermined outer diameter, the first reduction mode for linearly reducing the density of the porous glass layer and after the base material reaches the predetermined outer diameter Has found that a favorable result can be obtained by setting the second reduction mode to reduce linearly more gently than the first reduction mode.
[0013]
Furthermore, since the growth of the base material depends on the supply amount of the raw material gas from the burner for glass fine particle synthesis, that is, the size of the raw material gas outlet of the burner, the outer diameter (D _g ) of the base material and the raw material gas From the ratio (D _g / D _s ) of the size (D _s ) of the outlet, the conversion point of the density reduction rate in the porous glass layer, that is, the conversion point between the first reduction mode and the second reduction mode. I found out that is required. Furthermore, it was also found that if this ratio (D _g / D _s ) is adjusted so that 14 ≦ D _g / D _s ≦ 36, good results can be obtained.
[0014]
The present invention has been made from such a viewpoint. Basically, by switching from the first reduction mode to the second reduction mode in the process of depositing the glass fine particles, as a result, It is an object of the present invention to provide a method for producing a porous preform for an optical fiber in which better growth of the preform can be obtained.
[0015]
[Means for Solving the Problems]
The present invention according to claim 1 is a method for producing a porous optical fiber preform, in which glass particulates from a glass particulate synthesis burner are deposited on the outer periphery of a target member to form a porous preform for optical fibers,
When the density change of the porous glass layer according to the outer diameter of the optical fiber porous preform is linearly approximated, the outer diameter and the weight of the optical fiber porous preform growing due to the deposition of the glass fine particles are Monitoring, calculating the density of the porous glass layer, and linearly reducing the density of the porous glass layer at a constant rate until the porous optical fiber preform reaches a predetermined outer diameter . reduction while depositing the glass particles in mode, a predetermined after reaching the outer diameter, the glass in a second reduction mode for reducing the density of the porous glass layer was further loosely linearly than reduce the rate of the constant In addition to depositing fine particles, the conversion from the first reduction mode to the second reduction mode is made at a point where both approximate straight lines formed by both reduction modes intersect, and outside the porous preform for the optical fiber. Diameter ( _Dg ) And the size of the raw material gas outlet of the burner for glass fine particle synthesis (D _s ) (D _g / D _s ) Is 14 ≦ D _g / D _s The manufacturing method of the optical fiber porous preform is characterized by adjusting so that ≦ 36 .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of a production apparatus system for carrying out the method for producing an optical fiber porous preform according to the present invention, and FIG. 2 shows an example of a glass particle synthesis burner used in this production apparatus system. It is shown.
[0017]
In the present invention, both ends of the target member 10 are rotated while being held by gripping portions 20, 20 such as a chuck, while two glass particle synthesis burners 100 are opposed to the target member 10, for example. Are relatively moved to deposit and grow glass particles (soot) 11 from the glass particle synthesizing burner 100 on the outer periphery of the target member.
[0018]
The structure of the glass fine particle synthesizing burner 100 is not particularly limited. In the case of FIG. 2, a raw material gas flow to which a raw material gas such as SiCl ₄ (usually a carrier gas such as oxygen gas is often added) is supplied. An outlet 110, an inert gas outlet 120 to which an inert gas such as nitrogen gas is supplied, a combustible gas outlet 130 to which a combustible gas such as hydrogen gas is supplied, and the combustible gas outlet 130 are disposed. The plurality of combustion-supporting gas outlets 140 to which a combustion-supporting gas such as oxygen gas is supplied and the outermost inert gas outlet 150 to which an inert gas such as nitrogen gas is supplied.
[0019]
The flame 100a from the glass fine particle synthesizing burner 100 having such a structure is sprayed on the target member 10 side, and the glass fine particles 11 are deposited on the outer periphery thereof. At this time, in the present invention, as shown in FIG. Furthermore, the density change of the porous glass layer according to the outer diameter of the optical fiber porous preform is performed in a two-stage mode that is linearly approximated. More specifically, from the initial stage of deposition until the base material reaches a predetermined outer diameter, the density of the porous glass layer, which is a deposition layer of the glass fine particles 11, is linearly reduced at a constant rate. That is, the density of the porous glass layer is reduced along the virtual straight line I that substantially corresponds to the first reduction mode. And after a base material reaches a predetermined | prescribed outer diameter, the density of a porous glass layer is linearly reduced more loosely than the said fixed reduction rate. That is, the density of the porous glass layer is reduced substantially along the virtual straight line II corresponding to the second reduction mode. Here, the imaginary straight lines I to II are only ideal density control, and may be approximate values of the density along these imaginary straight lines I to II in actual control. .
[0020]
In the present invention, the density of the porous glass layer is calculated appropriately by a control device built in the computer while monitoring the growth of the base material, that is, its outer diameter by optical means, and also monitoring the weight of the base material. Seeking.
[0021]
In the case of FIG. 3, the density of the porous glass layer is set to about 0.75 at the initial stage of deposition, and is linearly reduced to about 0.25 until the outer diameter of the base material is about 70 mm. Until the final outer diameter of about 230 mm is reached, the density is gradually and linearly reduced between about 0.25 and 0.20. However, the present invention is not particularly limited to this. . That is, it can be controlled with a certain width by the difference in various parameters such as the final size of the base material outer diameter, the size of the source gas outlet 110 in the glass fine particle synthesis burner 100, and the source gas outlet rate. it can.
[0022]
As described above, the conversion point (inflection point C) for converting the density of the porous glass layer from the first reduction mode to the second reduction mode is, as shown in FIG. However, this is also considered to be adjusted due to the difference in various manufacturing parameters as described above. With respect to this point, the present invention and the like grow and deposit growth of the size (D _s ) of the source gas outlet 110 of the glass fine particle synthesis burner 100, that is, the spread of the source gas due to this size (D _s ). As a result of paying attention to the outer diameter (D _g ) of the base material and conducting various test studies, it was found that the outer diameter (D _g ) was obtained from the ratio (D _g / D _s ), as will be apparent from the examples described later. .
[0023]
Further, it was also found that if this ratio (D _g / D _s ) is adjusted so as to satisfy 14 ≦ D _g / D _s ≦ 36, good results can be obtained. That is, when D _g / D _s is less than 14, the outer diameter (D _g ) of the base material is too small for the spread of the raw material gas, so that the thermophoresis effect described above cannot be expected so much and the base material grows. This is because the deposition efficiency of the glass fine particles is lowered as a result. On the other hand, when D _g / D _s exceeds 36, the density difference between the porous glass layer at the center and the outer periphery of the base material becomes too large. As a result, as described above, soot cracking, This is because problems such as residual bubbles and delamination after transparent vitrification occur. Of course, when the density of the porous glass layer is lowered, the outer diameter of the base material is rapidly increased, which causes a problem that it is difficult to handle in an existing manufacturing apparatus system.
[0024]
For this reason, in the present invention, up to the conversion point of the density of the porous glass layer, in accordance with the first reduction mode, various gas flow rates in the glass fine particle synthesis burner 100 are adjusted, The outer diameter of the base material is grown to a predetermined outer diameter as quickly as possible. Thereafter, various gas flow rates in the glass fine particle synthesizing burner 100 are adjusted so as to conform to the second reduction mode, and grown to the final outer diameter of the base material. Accordingly, as seen from the whole manufacturing process, the final average deposition efficiency and average deposition rate of the glass fine particles are improved as described later, and as a result, excellent productivity is obtained.
[0025]
<Example 1>
Using the manufacturing apparatus system shown in FIG. 1 and the glass fine particle synthesis burner having the same structure as that shown in FIG. 2, 15 kg of SiO ₂ glass fine particles are deposited on the outer periphery of a target member having an outer diameter (diameter) of 30 mmφ. A porous base material was obtained.
[0026]
At this time, the size (D _s ) of the raw material gas outlet in the glass fine particle synthesis burner is 5.0 mm, the raw material gas is SiCl ₄ , the carrier gas is oxygen gas, the combustible gas is hydrogen gas, and the combustion supporting gas is Nitrogen gas was used as oxygen gas and inert gas, respectively. From the source gas outlet, the mixed gas flow rate of the source gas and the carrier gas was adjusted so that the gas flow rate was 9.8 m / sec. On the other hand, the flow rate as the fuel gas is such that the flow rate of the combustible gas is 1.3 to 1.7 m / sec, and the flow rate of the combustion-supporting gas is 10 to 13.0 m / sec. Adjusted. The flow rate of the inert gas was adjusted so that the flow rate was 0.7 m / sec.
[0027]
During this adjustment, the density of the porous glass layer and the above ratio (D _g / D _s ) are always calculated from the monitor information of the outer diameter of the base material and the weight of the base material, and the outer diameter of the base material from the initial stage of deposition. Until the predetermined outer diameter is reached, the density of the porous glass layer is reduced along the virtual straight line I corresponding to the first reduction mode as described above, while after reaching the predetermined outer diameter, As described above, the density of the porous glass layer was reduced along the virtual straight line II corresponding to the second reduction mode. At this time, the conversion point of the density of the porous glass layer was performed at D _g / D _s = 20.2.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 27.3%, and continued to increase steadily. The final average deposition efficiency was 68%, and the average deposition rate was 28.0 g / min. .
[0028]
<Example 2>
Deposition is performed under the same conditions as in Example 1 above, and the conversion point of the density of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating D _g / D _s = 14.3. I went there.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 25.5%, and showed a steady increase thereafter, the final average deposition efficiency was 65%, and the average deposition rate was 26.3 g / min. .
[0029]
<Example 3>
Deposition is performed under the same conditions as in Example 1 above, and the conversion point of the density of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating D _g / D _s = 35.3. I went there.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 24.2%, and then showed a steady increase. The final average deposition efficiency was 60.5%, and the average deposition rate was 23.8 g / min. there were.
[0030]
<Comparative example 1>
Using the manufacturing apparatus system shown in FIG. 1 and the glass fine particle synthesis burner having the same structure as that shown in FIG. 2, 15 kg of SiO ₂ glass fine particles are deposited on the outer periphery of a target member having an outer diameter (diameter) of 30 mmφ. A porous base material was obtained.
[0031]
At this time, the size (D _s ) of the raw material gas outlet in the glass fine particle synthesis burner is 5.0 mm, the raw material gas is SiCl ₄ , the carrier gas is oxygen gas, the combustible gas is hydrogen gas, and the combustion supporting gas is Nitrogen gas was used as oxygen gas and inert gas, respectively. From the source gas outlet, the mixed gas flow rate of the source gas and the carrier gas was adjusted so that the gas flow rate was 9.8 m / sec. On the other hand, the flow rate of the fuel gas is such that the flow rate of the combustible gas is 1.5 to 2.1 m / sec, and the flow rate of the combustible gas is 11.7 to 14.8 m / sec. Adjusted. The flow rate of the inert gas was adjusted so that the flow rate was 0.7 m / sec.
[0032]
During this adjustment, the density of the porous glass layer and the above ratio (D _g / D _s ) are always calculated from the monitor information of the outer diameter of the base material and the weight of the base material, and the outer diameter of the base material from the initial stage of deposition. Until the predetermined outer diameter is reached, the density of the porous glass layer is reduced along the virtual straight line I corresponding to the first reduction mode as described above, while after reaching the predetermined outer diameter, As described above, the density of the porous glass layer was reduced along the virtual straight line II corresponding to the second reduction mode. At this time, the conversion point of the density of the porous glass layer was performed at D _g / D _s = 12.8.
As a result, the deposition efficiency immediately after the start of deposition of the glass fine particles showed 22.8%, the growth of the base material was slow, the final average deposition efficiency was 50%, and the average deposition rate was 22.0 g / min.
[0033]
<Comparative example 2>
Deposition is performed under the same conditions as in Comparative Example 1, and the conversion point of the density of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating D _g / D _s = 39.8. I went there.
As a result, the deposition efficiency immediately after the start of deposition of the glass fine particles showed 23.8%, but the density difference between the central portion and the outer peripheral portion of the base material became too large, soot cracks and bubbles after transparent vitrification. Residual and delamination were observed.
[0034]
<Example 4>
Using the manufacturing apparatus system shown in FIG. 1 and the glass fine particle synthesis burner having the same structure as that shown in FIG. 2, 15 kg of SiO ₂ glass fine particles are deposited on the outer periphery of a target member having an outer diameter (diameter) of 30 mmφ. A porous base material was obtained.
[0035]
At this time, the size (D _s ) of the source gas outlet in the glass fine particle synthesis burner is 3.0 mm, the source gas is SiCl ₄ , the carrier gas is oxygen gas, the combustible gas is hydrogen gas, and the combustion supporting gas is Nitrogen gas was used as oxygen gas and inert gas, respectively. From the source gas outlet, the mixed gas flow rate of the source gas and the carrier gas was adjusted so that the gas flow rate was 24.3 m / sec. On the other hand, the flow rate of the fuel gas is such that the flow rate of the combustible gas is 1.4 to 2.1 m / sec, and the flow rate of the combustible gas is 13.4 to 17.5 m / sec. Adjusted. The flow rate of the inert gas was adjusted so that the flow rate was 0.7 m / sec.
[0036]
During this adjustment, the density of the porous glass layer and the above ratio (D _g / D _s ) are always calculated from the monitor information of the outer diameter of the base material and the weight of the base material, and the outer diameter of the base material from the initial stage of deposition. Until the predetermined outer diameter is reached, the density of the porous glass layer is reduced along the virtual straight line I corresponding to the first reduction mode as described above, while after reaching the predetermined outer diameter, As described above, the density of the porous glass layer was reduced along the virtual straight line II corresponding to the second reduction mode. At this time, the conversion point of the density of the porous glass layer was performed at D _g / D _s = 21.3.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 28.4%, and showed a steady increase thereafter, the final average deposition efficiency was 61.8%, and the average deposition rate was 25.5 g / min. there were.
[0037]
<Example 5>
Deposition is performed under the same conditions as in Example 4 above. The density conversion point of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating D _g / D _s = 14.8. I went there.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 28.7%, and continued to increase steadily. The final average deposition efficiency was 63%, and the average deposition rate was 25.2 g / min. .
[0038]
<Example 6>
Deposition is performed under the same conditions as in Example 4 above, and the conversion point of the density of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating D _g / D _s = 35.7. I went there.
As a result, the deposition efficiency immediately after the start of the deposition of the glass fine particles showed 27.8%, and showed a steady increase thereafter, the final average deposition efficiency was 60%, and the average deposition rate was 23.8 g / min. .
[0039]
<Comparative Example 3>
Using the manufacturing apparatus system shown in FIG. 1 and the glass fine particle synthesis burner having the same structure as that shown in FIG. 2, 15 kg of SiO ₂ glass fine particles are deposited on the outer periphery of a target member having an outer diameter (diameter) of 30 mmφ. A porous base material was obtained.
[0040]
At this time, the size (D _s ) of the source gas outlet in the glass fine particle synthesis burner is 3.0 mm, the source gas is SiCl ₄ , the carrier gas is oxygen gas, the combustible gas is hydrogen gas, and the combustion supporting gas is Nitrogen gas was used as oxygen gas and inert gas, respectively. From the source gas outlet, the mixed gas flow rate of the source gas and the carrier gas was adjusted so that the gas flow rate was 24.3 m / sec. On the other hand, the flow rate as the fuel gas was adjusted so that the flow rate of the combustible gas was 1.72 m / sec and the flow rate of the combustion-supporting gas was 16.2 m / sec. The flow rate of the inert gas was adjusted so that the flow rate was 0.7 m / sec.
[0041]
During this adjustment, the density of the porous glass layer and the above ratio (D _g / D _s ) are always calculated from the monitor information of the outer diameter of the base material and the weight of the base material, and the outer diameter of the base material from the initial stage of deposition. Until the predetermined outer diameter is reached, the density of the porous glass layer is reduced along the virtual straight line I corresponding to the first reduction mode as described above, while after reaching the predetermined outer diameter, As described above, the density of the porous glass layer was reduced along the virtual straight line II corresponding to the second reduction mode. At this time, the conversion point of the density of the porous glass layer was performed at D _g / D _s = 12.0.
As a result, the deposition efficiency immediately after the start of deposition of the fine glass particles was 24.5%, the growth of the base material was slow, the final average deposition efficiency was 48%, and the average deposition rate was 20.2 g / min.
[0042]
<Comparative example 4>
Deposition is performed under the same conditions as in Comparative Example 3, and the conversion point of the density of the porous glass layer at this time is calculated from the monitor information similar to the above by calculating from D _g / D _s = 39.2. I went there.
As a result, the deposition efficiency immediately after the start of deposition of the glass fine particles showed 23.8%, but the density difference between the central portion and the outer peripheral portion of the base material became too large, soot cracks and bubbles after transparent vitrification. Residual and delamination were observed.
[0043]
【The invention's effect】
As is clear from the above description, according to the method for manufacturing a porous preform for optical fiber according to the present invention, the density change of the porous glass layer according to the outer diameter of the porous preform for optical fiber is linearly approximated. In this case, until the porous optical fiber preform reaches a predetermined outer diameter, the density of the porous glass layer is linearly reduced at a constant rate from the first reduction mode to the predetermined outer diameter. After reaching, the density of the porous glass layer is reduced to a second reduction mode that linearly reduces the density more slowly than the fixed reduction rate. The average deposition efficiency and the average deposition rate are improved, so that excellent productivity can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing an example of a manufacturing apparatus system for carrying out a method for manufacturing a porous preform for an optical fiber according to the present invention.
FIG. 2 is an end view showing an example of a glass fine particle synthesis burner used in the manufacturing apparatus system of FIG. 1;
FIG. 3 is a schematic explanatory view showing an example of controlling the density of a porous glass layer in the method for producing a porous preform for an optical fiber according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Target member 11 Glass particulate 20 Grasping part 100 Glass particulate synthesis burner 100a Flame 110 Raw material gas outlet

Claims

A method for producing a porous optical fiber preform, in which glass particulates from a glass particulate synthesis burner are deposited on the outer periphery of a target member to form a porous preform for optical fibers,
When the density change of the porous glass layer according to the outer diameter of the optical fiber porous preform is linearly approximated, the outer diameter and the weight of the optical fiber porous preform growing due to the deposition of the glass fine particles are Monitoring, calculating the density of the porous glass layer, and linearly reducing the density of the porous glass layer at a constant rate until the porous optical fiber preform reaches a predetermined outer diameter . reduction while depositing the glass particles in mode, a predetermined after reaching the outer diameter, the glass in a second reduction mode for reducing the density of the porous glass layer was further loosely linearly than reduce the rate of the constant In addition to depositing fine particles, the conversion from the first reduction mode to the second reduction mode is made at a point where both approximate straight lines formed by both reduction modes intersect, and outside the porous preform for the optical fiber. Diameter ( _Dg ) And the size of the raw material gas outlet of the burner for glass fine particle synthesis (D _s ) (D _g / D _s ) Is 14 ≦ D _g / D _s It adjusts so that it may become <= 36, The manufacturing method of the porous preform | base_material for optical fibers characterized by the above-mentioned .