Patent Description:
The glass base material for optical fiber is produced by sintering the porous glass base material. The porous glass base material to be sintered is made by depositing silica fine particles on a starting material and is manufactured by VAD or OVD methods.

The silica fine particles deposited on the starting material are formed by burning the organic siloxane raw material in a burner (see, e.g., <CIT>, <CIT>, <CIT> and <CIT>). <CIT> discloses pressurizing the organic raw material, removing dissolved gas from the pressurized material, thereafter controlling the flow rate of the material by a mass flow controller and vaporizing the raw material.

One method for forming silica fine particles is to use a liquid organic siloxane raw material (hereinafter referred to as a liquid raw material), such as octamethylcyclotetrasiloxane (OMCTS) (see, e.g., <CIT>). In the method of <CIT>, silica fine particles are formed via steps of supplying the liquid raw material to a vaporizer, vaporizing the liquid raw material in the vaporizer to form a raw material gas, and burning the liquid raw material vaporized in a burner. In the method of <CIT>, the liquid raw material supplied to the vaporizer is controlled by a mass flow controller. <CIT> discloses an apparatus for feeding stock liquid for porous glass preform comprising an inert gas introducing pipe connected to a raw material liquid storage container. Raw material liquid is fed to a vaporizer connected to a burner. <CIT> discloses a quartz glass manufacturing apparatus comprising a liquid source cylinder and a vaporizer, both connected by a liquid source pipe having a flow control device. A pressure fluctuation buffering mechanism is further provided between the liquid source cylinder and the flow control device. The vaporizer is connected to a synthesis furnace, in which a burner is disposed, via a source gas pipe.

However, in the method of <CIT>, the flow rate of the liquid raw material may fluctuate, for example, when the gas dissolves in the liquid raw material and bubbles of the dissolved gas are mixed in. As the flow rate of the supplied liquid raw material fluctuates, the amount of liquid raw material vaporization in the vaporizer also fluctuates accordingly, and the amount of raw material gas supplied to the burner fluctuates. When the amount of raw material gas supplied to the burner fluctuates, the combustion reaction in the burner becomes unstable and the amount of formation of silica fine particles fluctuates. As a result, the density of silica fine particles becomes non-uniform, which leads to lower manufacturing efficiency and defects such as poor dehydration and bubbles in the glass during the sintering of the resulting porous glass base material for optical fibers into transparent glass.

The present invention was made in view of the above problem, and it is an object of the present invention to suppress a flow rate fluctuation of the liquid raw material of the organic siloxane supplied to the vaporizer and to uniformize a deposition density of the silica fine particles.

In order to solve the above problem, in the method of manufacturing the porous glass base material according to the present invention, a liquid organic siloxane raw material stored in a raw material tank of internal pressure P<NUM> is controlled by a mass flow controller at a predetermined flow rate and pumped through a pipe of internal pressure P<NUM> to a vaporizer, the liquid raw material is vaporized in the vaporizer and supplied as a gas raw material to a burner, and the silica fine particles formed by burning the gas raw material in the burner are deposited to form a porous glass base material. The present invention is characterized by the method of manufacture described above, wherein P<NUM> ≤ P<NUM> is satisfied.

It is preferable that the above manufacturing method includes a step of pumping the liquid raw material in the raw material tank to the mass flow controller using a liquid feed pump. And when the pressure of the liquid raw material supplied to the mass flow controller is P<NUM>, it is preferable that P<NUM> ≤ P<NUM> < P<NUM> is satisfied. In addition, in the step of pumping, it is more preferable that the liquid feed pump raises the pressure of the liquid raw material to P<NUM>, and supplies the liquid raw material to the mass flow controller while reducing the pressure to P<NUM> via the pressure loss unit, so that P<NUM> ≤ P<NUM> < P<NUM> < P<NUM> is satisfied. In this case, it is preferable that P<NUM> ≤ <NUM>. 6P<NUM> is satisfied.

It is also preferable to return some or all of the liquid raw material pressurized to P<NUM> by the liquid feed pump to the raw material tank and supply the remainder to the mass flow controller.

In the present invention, P<NUM> ≤ <NUM> MPa is preferable, and P<NUM> ≤ <NUM> MPa is more preferable.

In the present invention, it is preferable to heat and keep the pipe through which the liquid raw material flows to maintain a temperature above the freezing point of the liquid raw material. In the present invention, the liquid raw material may be octamethylcyclotetrasiloxane (OMCTS).

A manufacturing apparatus of porous glass base material for optical fiber comprises: a raw material tank suitable for storing a liquid raw material, which is organic siloxane in a liquid state and filling the remaining space with inert gas; a liquid feed pump configured to pump the liquid raw material from the raw material tank; a circulating pipe configured to return part or all of the liquid raw material pumped by the liquid feed pump to the raw material tank; a first supply pipe branched from the circulating pipe; a pressure loss unit provided downstream of the first supply pipe; a second supply pipe provided downstream of the pressure loss unit; a mass flow controller provided downstream of the pressure loss unit via the second supply pipe configured to control the flow rate of the liquid raw material to a predetermined flow value; a third supply pipe provided downstream of the mass flow controller; a vaporizer provided downstream of the mass flow controller via the third supply pipe, configured to vaporize the liquid raw material; and a burner configured to combust the raw material gas vaporized by the vaporizer to deposit silica fine particles. In use, when the internal pressure of the raw material tank is P<NUM>, the internal pressure of the circulating pipe is P<NUM>, the internal pressure of the second supply pipe is P<NUM>, and the internal pressure of the third supply pipe is P<NUM>, P<NUM> ≤ P<NUM> < P<NUM> < P<NUM> is satisfied.

In the present invention, it is preferable to provide heating/warmth-keeping unit for heating and keeping the circulating pipe, the first supply pipe, the second supply pipe, and the third supply pipe at a temperature above the freezing point of the liquid raw material.

According to the present invention, with respect to organic siloxane raw materials typified by octamethylcyclotetrasiloxane (OMCTS), it is possible to suppress the flow rate fluctuation in the liquid raw material supplied to the vaporizer and realize stable supply.

Hereinafter, based on the embodiment, the present invention will be described in more detail. In the following description, portions already described are denoted by the same reference numerals, and description of the portion once described will be omitted accordingly.

<FIG> illustrates the raw material supply flow of the porous glass base material manufacturing apparatus for optical fiber. The porous glass matrix manufacturing apparatus for optical fiber shown in <FIG> has a raw material tank <NUM>, a liquid feed pump <NUM>, an accumulator <NUM>, a pressure loss unit <NUM>, <NUM>, <NUM>, a mass flow controller <NUM>, a vaporizer <NUM>, a burner <NUM>, and an intermediate container <NUM>, which are connected by piping.

The organic siloxane raw material in liquid form (hereinafter simply referred to as "liquid raw material") <NUM> is supplied from the raw material injection pipe <NUM> and stored in the raw material tank <NUM>. At this time, the internal pressure of the raw material tank is set to P<NUM>. The liquid raw material <NUM> stored in the raw material tank <NUM> is pressurized by the liquid feed pump <NUM> and pumped to the pressure loss unit <NUM> through a pipe <NUM>. Here, let P<NUM> be the internal pressure in the pipe <NUM> that supplies liquid raw material <NUM> to the pressure loss unit <NUM>.

The pressure loss unit <NUM> de-pressurizes the supplied liquid raw material and supplies the same to the mass flow controller <NUM> via a pipe <NUM>. Here, let P<NUM> be the internal pressure in the pipe <NUM> that supplies liquid raw material <NUM> to the mass flow controller <NUM>.

The mass flow controller <NUM> controls the supplied liquid raw material <NUM> to a predetermined flow rate and supplies the liquid raw material <NUM> to the vaporizer <NUM> through a pipe <NUM>. Here, let P<NUM> be the internal pressure of the pipe <NUM>.

The vaporizer <NUM> vaporizes the supplied liquid raw material <NUM> to make the raw material gas <NUM>. The vaporized raw material gas <NUM> is de-pressurized as it passes through a pipe <NUM> and supplied to the burner <NUM>. The burner <NUM> combusts the raw material gas <NUM> to produce silica fine particles <NUM>, which are deposited on the starting material (not shown) by the combustion reaction. As described above, the porous glass base material for optical fibers can be manufactured.

In the manufacturing method using the porous glass base material manufacturing apparatus for optical fibers as described above, if gas bubbles are mixed in the liquid raw material <NUM> flowing through the pipe <NUM>, the actual flow rate of the liquid raw material <NUM> supplied to the vaporizer <NUM> will fluctuate. As a result, the amount of liquid raw material <NUM> vaporized in the vaporizer <NUM> (i.e., the amount of the raw material gas <NUM> generated) fluctuates and becomes unstable, and the flow rate of raw material gas <NUM> flowing through the pipe <NUM> also becomes unstable. If the flow rate of raw material gas <NUM> flowing through the pipe <NUM> fluctuates, the production rate of silica fine particles <NUM> in the burner <NUM> becomes unstable, so the density of silica fine particles <NUM> deposited on the starting material fluctuates and becomes uneven. As a result, the density and shape of the produced porous glass base material becomes uneven.

Therefore, it is preferable to take out and pump the liquid raw material <NUM> from the bottom of the raw material tank <NUM> where far enough away from the liquid surface of the liquid raw material <NUM> stored in the raw material tank <NUM> to prevent as much as possible air bubbles from mixing with the liquid raw material <NUM> that is pumped from the raw material tank <NUM>.

However, it is inevitable that some of the gases <NUM> in contact with the liquid surface of the liquid raw material <NUM> stored in the raw material tank <NUM> will dissolve in the liquid raw material <NUM>. This dissolved gas <NUM> may foam in the pipe on the way to the vaporizer <NUM> and cause instability in the amount of the raw material gas produced in the vaporizer <NUM>.

Therefore, in the porous glass matrix manufacturing device for optical fibers according to the present invention, liquid raw material <NUM> taken out from the raw material tank <NUM> is pressurized by the liquid feed pump <NUM>. Here, the internal pressure P<NUM> of the pipe <NUM>, which is located downstream of the mass flow controller <NUM> and supplies liquid raw material <NUM> to the vaporizer <NUM>, should not be less than the internal pressure P<NUM> of the raw material tank <NUM> (i.e., P<NUM> ≤ P<NUM>). Specifically, if the internal pressure P<NUM> of the raw material tank <NUM> is set to be higher than atmospheric pressure, P<NUM> is adjusted by supplying inert gas regulated by a pressure regulator from the inert gas supply pipe <NUM>. The discharge pressure of the liquid feed pump <NUM> is adjusted according to the pressure loss in the flow path from the liquid feed pump <NUM> to the pipe <NUM>, and the pressure loss unit <NUM>, such as a needle valve, is provided in the middle of the flow path from the pipe <NUM> to the burner <NUM> to adjust the pressure. By these methods, P<NUM> is adjusted so as to be sufficiently higher than the atmospheric pressure at the outlet of the burner <NUM> and not to be smaller than the internal pressure P<NUM> of the raw material tank <NUM>. In this way, the gas <NUM> dissolving in the liquid raw material <NUM> can be effectively suppressed from foaming in the pipe <NUM>.

Further, if a liquid containing air bubbles is flowed through the mass flow controller <NUM>, accurate flow rate measurement becomes difficult, and the flow rate adjustment operation of the mass flow controller <NUM> may become unstable. Therefore, in addition to setting P<NUM> ≤ P<NUM>, the internal pressure P<NUM> of the pipe <NUM> located upstream of the mass flow controller <NUM> should not be less than the internal pressure P<NUM> of the raw material tank <NUM> (i.e., P<NUM> ≤ P<NUM>). Specifically, the internal pressure P<NUM> of the raw material tank <NUM> is adjusted by supplying inert gas regulated by a pressure regulator from the inert gas supply pipe <NUM>. Also, by adjusting the discharge pressure of the liquid feed pump <NUM> according to the pressure loss in the flow path from the liquid feed pump <NUM> to the mass flow controller <NUM>, the internal pressure P<NUM> of the pipe <NUM> is adjusted so that the internal pressure P<NUM> is not less than the internal pressure P<NUM> of the raw material tank <NUM>. In addition, a pressure loss unit <NUM>, such as a needle valve, is provided in the middle of the flow path from the mass flow controller <NUM> to the burner <NUM> to adjust the pressure. By these methods, P<NUM> is adjusted so as to be sufficiently higher than the atmospheric pressure at the outlet of the burner <NUM> and not to be smaller than the internal pressure P<NUM> of the raw material tank <NUM>. In this way, air bubbles can be inhibited from entering the liquid raw material <NUM> that passes through the mass flow controller <NUM>.

Here, it is preferable to make the internal pressure P<NUM> upstream (pipe <NUM> side) of the mass flow controller <NUM> higher than the internal pressure P<NUM> downstream (pipe <NUM> side) (i.e., P<NUM> ≤ P<NUM> < P<NUM>). In this way, the flow rate adjustment operation of the mass flow controller <NUM> is further stabilized. In particular, setting P<NUM> to a pressure higher than P<NUM> by <NUM> MPa or more is preferable because the flow rate adjustment operation of the mass flow controller <NUM> is stable. To realize such relationship between P<NUM> and P<NUM>, the power of the liquid feed pump <NUM> may be adjusted so that the discharge pressure of the liquid feed pump <NUM> is sufficiently high according to the pressure loss in the flow path from the liquid feed pump <NUM> to the mass flow controller <NUM>.

In addition, in the manufacturing method using the porous glass base material manufacturing apparatus for optical fibers as described above, the flow rate of liquid raw material <NUM> may fluctuate depending on the operation of the liquid feed pump <NUM>. Namely, if the pressure upstream of the mass flow controller <NUM> fluctuates in a short period of time due to fluctuations in the discharge pressure of the pump with the internal motion of the pump when the liquid feed pump <NUM> pressurizes the liquid raw material <NUM>, the flow rate adjustment by the mass flow controller <NUM> may not be able to keep up with the pressure fluctuations. Therefore, the pressure loss unit <NUM> (e.g., pressure reducing valve, orifice, etc.) should be provided between the pipe <NUM>, where the liquid raw material <NUM> is discharged from the liquid feed pump <NUM>, and the pipe <NUM>, which supplies the liquid raw material <NUM> to the mass flow controller <NUM>, so that the internal pressure P<NUM> of the pipe <NUM> is higher than the internal pressure P<NUM> of the pipe <NUM> (i.e., P<NUM> < P<NUM>). In this way, the internal pressure P<NUM> of the pipe <NUM> is less susceptible to fluctuations in the discharge pressure of the liquid feed pump <NUM>, and the flow rate adjustment operation by the mass flow controller <NUM> can be stabilized. In particular, if the pressure loss unit <NUM> is set so that the pressure of P<NUM> is approximately <NUM> times or less than P<NUM>, it is preferable because the fluctuations of P<NUM> can be effectively suppressed. Summarizing the above, it is preferable that P1 ≤ P2 <P3 <P4.

The end of the raw material injection pipe <NUM>, which injects the liquid raw material <NUM> into the raw material tank <NUM>, is installed so that the end of the raw material injection pipe <NUM> is below the liquid surface of the liquid raw material <NUM> stored in the raw material tank <NUM>. In this way, it is possible to prevent the gas <NUM> existing in the space above the liquid surface of the liquid raw material <NUM> from being swallowed up and the bubbles of the gas <NUM> from being mixed into the liquid raw material <NUM>.

When a highly flammable liquid raw material such as octamethylcyclotetrasiloxane (OMCTS) is used, the gas <NUM> in the space above the liquid surface of the liquid raw material <NUM> in the raw material tank <NUM> may be an inert gas, for example, nitrogen, argon, helium, etc. In this way, unintended oxidation reactions in the raw material tank <NUM> can be prevented. To supply these inert gases to the upper space of the raw material tank <NUM>, an inert gas supply pipe <NUM> may be provided, as shown in <FIG>.

The internal pressure P<NUM> of the raw material tank <NUM> may be maintained at a more positive pressure than atmospheric pressure. In this way, even if the raw material tank <NUM> has unintentional pinholes, etc., the outside air with oxygen can be prevented from flowing into the raw material tank <NUM>.

On the other hand, it is preferable to reduce the internal pressure P<NUM> of the raw material tank <NUM> and the pressure fluctuation of P<NUM> to prevent excessive dissolution of the gas <NUM> into the liquid raw material <NUM> stored in the raw material tank <NUM>. In particular, it is preferable to keep the gauge pressure of the internal pressure P<NUM> of the raw material tank <NUM> to <NUM> MPa or less, and even more preferable to keep it to <NUM> MPa or less. It is preferable to keep the pressure fluctuation of P<NUM> within ±<NUM> MPa, and it is even more preferable to keep it within ±<NUM> MPa.

When the liquid raw material <NUM> is vaporized in the vaporizer <NUM>, the gas <NUM> dissolved in the liquid raw material <NUM> is also released. The gas <NUM> in the upper space of the raw material tank <NUM> should have a constant gas species (in the case of a mixture of gases, each gas species and mixing ratio thereof), and the fluctuation range of the internal pressure P<NUM> of the raw material tank <NUM> should be small. In this way, the amount of gas <NUM> dissolved in the liquid raw material <NUM> is stabilized. As the amount of gas <NUM> dissolved in the liquid raw material <NUM> stabilizes, the partial pressure of the gas <NUM> released in the vaporizer <NUM> is also stabilized. As a result, the flow rate of raw material gas <NUM> supplied to the burner <NUM> can be stabilized.

To adjust the pressure fluctuation of the internal pressure P<NUM> in the raw material tank <NUM>, the pressure of the inert gas supplied from the inert gas supply pipe <NUM> may be adjusted with a pressure reducing valve (not shown) to maintain a constant pressure. A safety valve <NUM> and a back-pressure valve (not shown) may also be provided to maintain the internal pressure P<NUM> by de-pressurizing the raw material tank <NUM> so that the internal pressure P<NUM> falls below the predetermined pressure if the internal pressure P<NUM> unexpectedly exceeds the predetermined pressure.

As the liquid feed pump <NUM>, a diaphragm pump may be used as a metering pump. Otherwise, a plunger pump or a gear pump may be used. If the pulsation of the pressure P<NUM> in the pipe <NUM> by the pump <NUM> is large, the pressure P<NUM> in the pipe <NUM> may also pulsate therewith. In order to suppress the pulsation of P<NUM>, it is preferable to limit the fluctuation of P<NUM> to within ± <NUM> MPa, and it is even more preferable to limit it to within ± <NUM> MPa.

To suppress the pulsation of P<NUM>, a pulsation-free pump may be used, or an accumulator <NUM> may be installed between the discharge side of the pump <NUM> and the pipe <NUM>, and an orifice or other pressure loss unit may be installed. The accumulator <NUM> is a buffering device that suppresses the pulsation of the liquid by repeating the expansion and contraction of the diaphragm (rubber membrane) as the pulsating liquid passes through it.

As shown in <FIG>, a pipe <NUM> may be branched from the middle of the pipe <NUM>, and a pressure loss unit <NUM> such as orifice, safety valve, back pressure valve, needle valve, etc. may be attached therein to discharge part of the liquid raw material <NUM> pumped from the liquid feed pump <NUM> to the pipe <NUM>. In this way, the fluctuation range of the internal pressure P<NUM> of the raw material tank <NUM> can be kept small.

In addition, the discharged liquid raw material <NUM> may be held in an intermediate container <NUM> and left to stand, for example, to remove unintentionally mixed air bubbles, and then the liquid raw material <NUM> may be returned to the raw material tank <NUM> for reuse.

<FIG> shows a modification of the porous glass base material manufacturing apparatus for optical fiber shown in <FIG>. The porous glass base material manufacturing apparatus for optical fiber shown in <FIG> adopts a configuration in which liquid raw material <NUM> discharged through the pressure loss unit <NUM> is returned to the raw material tank <NUM> through a pipe <NUM>. The liquid raw material <NUM> pumped from the raw material tank <NUM> circulates through the liquid feed pump <NUM>, the pipe <NUM>, the pipe <NUM>, the pressure loss unit <NUM>, and the pipe <NUM>, and returns to the raw material tank <NUM>. In this way, the discharged liquid raw material <NUM> can be easily reused.

The end of the pipe <NUM> returning the discharged liquid raw material <NUM> to the raw material tank <NUM>, as in the case of raw material injection pipe <NUM>, is preferably installed below the liquid surface of liquid raw material <NUM> in the raw material tank <NUM>. With such a configuration, the liquid raw material <NUM> returning to the raw material tank <NUM> from the pipe <NUM> can prevent air bubbles from entering the liquid raw material <NUM> by entrapping the gas <NUM> that exists in the space above the liquid surface.

The present invention will be described in detail with Examples below. In the examples, the porous glass base material manufacturing apparatus for optical fiber with the configuration shown in <FIG> was used. As the liquid raw material, liquid octamethylcyclotetrasiloxane (OMCTS) was used.

Initially, liquid raw material <NUM> was supplied to the raw material tank <NUM> through the raw material injection pipe <NUM> and stored. The liquid raw material <NUM> stored in the raw material tank <NUM> was then pumped into the pipe <NUM> by the liquid feed pump <NUM>. A pipe <NUM> was branched from the middle of the pipe <NUM> to make a part of the liquid raw material <NUM>, which passed through the pipe <NUM> and an orifice as a pressure loss unit <NUM>, return to the raw material tank <NUM> through the pipe <NUM>.

On the other hand, the rest of the liquid raw material <NUM> pumped into the pipe <NUM> was de-pressurized by a pressure reducing valve as a pressure loss unit <NUM>, pumped into the pipe <NUM>, and was fed to the vaporizer <NUM> through the pipe <NUM> by controlling the flow rate with a mass flow controller <NUM>. The raw material tank <NUM> and the flow path from the raw material tank <NUM> to the vaporizer <NUM> were heated and kept warm as needed to maintain a temperature of <NUM>-<NUM>. It is preferable that the temperature range to be kept warm is above the freezing point of the liquid raw material <NUM> and below the flashpoint. When using OMCTS as the liquid raw material <NUM>, it is preferable to heat and keep the temperature between <NUM> and <NUM> because the freezing point of OMCTS is <NUM> and the flashpoint is <NUM>.

The space above the liquid surface of liquid raw material <NUM> in the raw material tank <NUM> was filled with nitrogen supplied from the inert gas supply pipe <NUM>. The internal pressure P<NUM> of the raw material tank <NUM> was maintained at an average of <NUM> MPa at gauge pressure, and the fluctuation range of P<NUM> during production (maximum value of P<NUM> - minimum value of P<NUM>) was kept within ± <NUM> MPa.

The discharge rate of the liquid feed pump <NUM> was set to <NUM> cc/min and the accumulator <NUM> was installed directly below the liquid feed pump <NUM>. The internal pressure P<NUM> of pipe <NUM> was kept at <NUM> ± <NUM> MPa at gauge pressure. The internal pressure P<NUM> of the pipe <NUM> was kept constant in the range of <NUM> to <NUM> MPa at gauge pressure, as shown in Examples <NUM>-<NUM> in Table <NUM> described later. The liquid raw material <NUM> was then fed by the mass flow controller <NUM> toward the vaporizer <NUM> at a flow rate in the range of <NUM> to <NUM>/min as shown in Examples <NUM>-<NUM>.

At the start-up of the apparatus, the liquid raw material <NUM> was circulated through the liquid feed pump <NUM>, pipe <NUM>, pipe <NUM>, and the pressure loss unit <NUM> (orifice), and was returned from pipe <NUM> to the raw material tank <NUM>, by operating the liquid feed pump <NUM> with the valve <NUM> installed upstream of the pressure loss unit <NUM> (pressure reducing valve) closed. By circulating the liquid raw material <NUM> in this way, the residual gas in the pipe can be pushed out and the pipe can be filled with bubble-free liquid raw material.

<FIG> illustrates the raw material supply flow around the vaporizer in the Examples. The liquid raw material <NUM> pumped into the pipe <NUM>, the flow rate of which was adjusted to a predetermined value (g/min) by the mass flow controller <NUM>, was supplied to the vaporizer <NUM> through the pipe <NUM>.

The temperature of the vaporizer <NUM> was set to <NUM>. From the point of view of efficiently vaporizing the raw material OMCTS and preventing polymerization reactions, it is preferable to set the temperature of the vaporizer <NUM> to <NUM>-<NUM>.

Nitrogen gas at a constant flow rate (<NUM>, <NUM> atm standard equivalent, L/min) heated to <NUM> in a heat exchanger as carrier gas <NUM> was supplied from a pipe <NUM> connected to the vaporizer <NUM>. In this way, the liquid raw material <NUM> and the carrier gas <NUM> were mixed in the vaporizer <NUM> to promote the vaporization of the liquid raw material <NUM>.

As the carrier gas <NUM>, in addition to nitrogen, an inert gas such as argon or helium, oxygen, or a mixed gas of oxygen and an inert gas may be used. The flow rate of carrier gas <NUM> was controlled by a mass flow controller (not shown). The carrier gas <NUM> was supplied by heating with a heat exchanger (not shown).

The raw material gas <NUM>, which is a mixture of the gas OMCTS obtained by vaporizing the liquid raw material <NUM> and nitrogen as the carrier gas <NUM>, was supplied to the burner <NUM> through pipe <NUM> and a needle valve as a pressure loss unit <NUM>. The pipe <NUM>, the pressure loss unit <NUM> (needle valve), and a pipe <NUM> were heated to <NUM> to prevent condensation of the raw gas <NUM>.

The raw material gas <NUM>, which passed through the pressure loss unit <NUM> (needle valve), was further mixed with a constant flow of oxygen gas <NUM> heated to <NUM> through a pipe <NUM>. This raw material gas <NUM> (a mixture of gas OMCTS and carrier gas) mixed with additional oxygen gas <NUM> was then supplied to the burner <NUM>.

From the viewpoint of preventing recondensation of the liquid raw material <NUM>, the oxygen gas <NUM> mixed here may be supplied at a state where heated in advance to a temperature higher than the liquefaction temperature expected from the partial pressure of the raw material gas <NUM> in the mixed gas by using a heat exchanger (not shown) or the like. By mixing oxygen with the raw material gas <NUM> in advance before supplying the burner <NUM>, the combustion reaction of the raw material gas <NUM> in the burner <NUM> can be promoted.

In addition to the mixture of the gas, the burner <NUM> is supplied with combustible gas for combustion, oxygen gas for combustion, and seal gas, if necessary. Hydrogen, methane, ethane, and propane can be used as combustible gas for combustion. As a seal gas, an inert gas such as nitrogen, argon, helium, etc., or oxygen or a mixture of oxygen and an inert gas is preferably used.

In the flame of the burner <NUM>, the raw material gas <NUM>, the combustible gas for combustion, the oxygen gas for combustion, and the like are mixed and burned to form silica fine particles <NUM>. The formed silica fine particles <NUM> were deposited on the starting material to form a porous glass matrix for optical fiber.

In addition, the porous glass base material was heated at <NUM> in a helium-containing atmosphere to manufacture a transparent glass base material for optical fiber.

The internal pressure P<NUM> of the raw material tank <NUM>, the internal pressure P<NUM> of the pipe <NUM>, the internal pressure P<NUM> of the pipe <NUM>, and the internal pressure P<NUM> of the pipe <NUM> were measured with pressure gauges as the gauge pressures in the above manufacturing method of the glass base material for optical fiber. (P<NUM>-P<NUM>) corresponds to the differential pressure before and after the mass flow controller <NUM>. The flow rate of the mixture of the raw material gas <NUM> and the carrier gas <NUM> passing through the pipe <NUM> was measured by a mass flow meter. Measurements were performed for <NUM> minutes and the fluctuation rate of the mass flow meter reading (= (max-min) / mean x <NUM>%) was measured. The mass flow meter used was a thermal type, and the measurement was performed by summing up the mixture of raw material gas <NUM> and the carrier gas <NUM> in terms of N<NUM> heat capacity. Actual flow conversions by conversion factor and other factors were not performed.

Comparative Example <NUM> is an example of manufacturing porous glass base material using the same equipment as in the example, with different conditions. In Comparative Example <NUM>, the porous glass base material was manufactured with the pressure loss unit <NUM> (needle valve) released and the internal pressure P<NUM> of the pipe <NUM> was set to <NUM> MPa at gauge pressure. In Comparative Example <NUM>, the internal pressure P<NUM> of the raw material tank <NUM> is <NUM> MPa, so the relationship between P<NUM> and P<NUM> is P<NUM> > P<NUM>. As a result, the fluctuation rate of the flow rate exceeded <NUM>%.

Comparative Example <NUM> is an example of manufacturing a porous glass base material with an apparatus without the liquid feed pump <NUM> as shown in <FIG>. In the apparatus having such a configuration, the liquid raw material <NUM> was supplied with the supply pressure of nitrogen supplied from the inert gas supply pipe <NUM> set to <NUM> MPa. As a result, the fluctuation rate of the flow rate exceeded <NUM>%.

The conditions and flow rate fluctuation for Examples <NUM>-<NUM> and Comparative Examples <NUM> and <NUM> are shown in Table <NUM>.

As shown as Examples <NUM>-<NUM> in Table <NUM>, it can be seen that by setting P<NUM> ≤ P<NUM>, fluctuation in the flow rate of the raw material gas supplied to the burner can be suppressed. Further, from the comparison between each Example and Comparative Example <NUM>, it can be seen that it is preferable that P<NUM> ≤ P<NUM> < P3.

Claim 1:
A manufacturing method of porous glass base material for optical fiber comprising:
pumping a liquid raw material (<NUM>) of an organic siloxane stored in a raw material tank (<NUM>) at internal pressure P<NUM> into a vaporizer (<NUM>) through a pipe at internal pressure P<NUM>, controlled by a mass flow controller (<NUM>) at a predetermined flow rate;
vaporizing the liquid raw material in the vaporizer (<NUM>) and supplying the vaporized raw material to the burner (<NUM>) as a gas raw material (<NUM>); and
forming a porous glass base material by depositing silica fine particles formed by burning the gas raw material (<NUM>) in the burner (<NUM>),
wherein P<NUM> ≤ P<NUM> is satisfied.